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Learn how to conduct fair workplace investigations that comply with ACAS guidance. Our practical guide covers the investigation process, interviewing witnesses, employee rights, confidentiality, and when to use an independent investigator.
Written by:Â Rachel Ford-Evans, Employment Law Partner, Darwin Gray | Last updated: 21/04/2026 | Reviewed by: Rachel Ford-Evans
When something goes wrong at work, whether it’s alleged misconduct, a grievance, a complaint of harassment, or a health and safety incident, you need to find out what happened before you decide what to do about it. That’s where workplace investigations come in.
A workplace investigation is a structured, fact-finding process. Done well, it gives you the information you need to make fair, defensible decisions. Done badly, it can lead to unfair dismissal claims, discrimination complaints, and serious damage to workplace trust.
The stakes are high. Employment tribunals will scrutinise how you conducted your investigation. If they find it was inadequate, any dismissal that follows is likely to be unfair, regardless of whether the employee actually did what you accused them of. The ACAS Code of Practice on disciplinary and grievance procedures makes clear that carrying out a reasonable investigation is a fundamental requirement of a fair process.
This guide explains how to conduct workplace investigations that are fair, thorough, and legally compliant.
A workplace investigation is a formal process of gathering and analysing evidence to establish the facts about a workplace issue. It’s not about proving someone guilty or justifying a decision you’ve already made. It’s about finding out what happened so you can make an informed decision.
Investigations typically involve:
The investigation is separate from any disciplinary hearing that might follow. The investigator establishes the facts; someone else decides what action to take.
You should consider conducting a formal investigation when:
Any allegation of misconduct that could result in disciplinary action requires investigation. This includes:
For gross misconduct allegations that could lead to dismissal, a thorough investigation is essential.
When an employee raises a formal grievance, you need to investigate to understand what happened and whether the complaint is justified. Common grievances include:
Complaints of bullying, harassment, or discrimination require particularly careful investigation. These cases often involve conflicting accounts and can result in significant legal claims if mishandled.
When an employee raises concerns about wrongdoing (a “protected disclosure”), you must take it seriously and investigate appropriately. Getting this wrong can lead to automatic unfair dismissal claims.
Workplace accidents and near-misses should be investigated to understand what happened and prevent recurrence.
Minor issues can often be resolved informally with a quiet conversation. The ACAS Code recognises that many potential disciplinary or grievance matters can be sorted out this way. Save formal investigations for matters that genuinely require them.
The ACAS Code of Practice on disciplinary and grievance procedures sets out the minimum standards employers must follow. While failing to follow the Code isn’t automatically unlawful, employment tribunals must take it into account when considering whether an employer acted reasonably.
Key requirements from the Code:
If a tribunal finds you unreasonably failed to follow the Code, it can increase any compensation award by up to 25%. That’s a significant financial penalty on top of whatever the original award might be.
Different situations call for different approaches.
These focus on alleged misconduct by an employee. The aim is to gather evidence about what happened so you can decide whether there’s a case to answer at a disciplinary hearing.
When an employee complains about their treatment, you investigate to understand their concerns and determine whether they’re justified. The ACAS Code allows the investigator and decision-maker roles to be combined in grievance cases, though separation may still be preferable for complex matters.
These require particular sensitivity. You’ll often be dealing with distressed complainants, disputed accounts, and allegations that are hard to prove. It’s important to protect both the complainant and the accused from unfair treatment during the process.
When someone raises concerns about wrongdoing in the organisation, you need to investigate the substance of their concerns, not just treat it as a grievance about how they’ve been treated.
These focus on what caused an incident and how to prevent it happening again. They may run alongside (but separately from) any disciplinary investigation if individual misconduct is suspected.
Allegations of financial wrongdoing often require specialist skills and may involve coordination with external auditors, regulators, or the police.
Before you start, think through what you’re dealing with:
Create terms of reference that set out the scope of the investigation, the issues to be examined, and the expected timeframe. This helps keep the investigation focused and provides clarity for everyone involved.
Choose someone who is:
For straightforward matters, a line manager or HR professional can often investigate. For complex or sensitive cases, consider an external investigator.
Collect relevant documents early, before memories fade or evidence disappears:
Keep a record of what you’ve collected, when, and from where.
If someone has raised a complaint, meet with them first to understand:
Take detailed notes. Let them tell their story in their own words before asking clarifying questions.
Identify anyone who might have relevant information and arrange to meet with them.
Some witnesses may be reluctant to get involved. Explore their concerns, offer reassurance about confidentiality, and explain that their evidence could be important. Don’t force anyone to participate, but note any refusals.
The employee who is the subject of allegations must have an opportunity to respond. Give them:
There’s no statutory right to be accompanied at an investigation meeting (unlike disciplinary hearings), but many employers allow it. Consider allowing a companion, particularly if the allegations are serious.
Listen to their response, ask clarifying questions, and follow up on any new issues they raise. Their account might lead you to other witnesses or evidence you hadn’t considered.
Once you’ve gathered the evidence, step back and consider:
In most workplace investigations, you’re working on the “balance of probabilities,” meaning what is more likely than not to have happened. You’re not trying to prove anything “beyond reasonable doubt.”
Consider evidence that supports and contradicts the allegations. An investigation that only looks for evidence of guilt isn’t a fair investigation.
Your report should include:
The report should be factual and evidence-based. It’s not the investigator’s job to decide guilt or recommend specific sanctions. That’s for the decision-maker at any subsequent hearing.
Present your report to whoever will be making decisions about next steps. This might be a manager conducting a disciplinary hearing, an HR team considering a grievance outcome, or a senior leader deciding on organisational action.
The decision-maker should consider your findings but reach their own conclusions. The investigation informs the decision; it doesn’t dictate it.
Interviews are at the heart of most investigations. Getting them right matters.
Confidentiality serves several purposes: it protects those involved from gossip and prejudgment, encourages witnesses to speak freely, and preserves the integrity of the process.
Absolute confidentiality isn’t always possible. You can’t promise a witness that nothing they say will ever be shared. Evidence may need to be disclosed to allow the accused to respond. Tribunal proceedings may require disclosure. Be honest about these limits from the start.
Sometimes witnesses ask to remain anonymous. Granting anonymity is problematic because it prevents the accused from properly challenging the evidence. Avoid it where possible, but if it’s genuinely necessary (for example, where there’s a real risk of intimidation), take extra steps to verify the evidence and give the accused a fair opportunity to respond to the substance of the allegations.
Employees who are subject to investigation have rights that you must respect.
Employees must be told what they’re accused of in enough detail to allow them to respond. Vague allegations like “inappropriate behaviour” aren’t enough. Be specific about what they’re alleged to have done, when, and to whom.
Before any decision is made, the employee must have an opportunity to give their side of the story. This usually happens at an investigation meeting and again at any disciplinary hearing.
There’s no statutory right to be accompanied at investigation meetings, but there is at disciplinary and grievance hearings. The companion can be a colleague or trade union representative. Consider allowing accompaniment at investigation meetings too, especially for serious allegations.
The overall process must be fair. That means reasonable timescales, impartial investigators, decisions based on evidence, and the opportunity to appeal any disciplinary outcome.
Employees have rights under UK GDPR regarding how their personal data is processed during investigations. You need a lawful basis for processing, you must be transparent about what you’re doing, and you must keep data secure. Employees can request access to their personal data, though you may be able to withhold some information (for example, to protect others).
Sometimes you need to suspend an employee while you investigate. This might be appropriate if:
Unnecessary or prolonged suspension can damage trust and lead to claims of breach of contract or constructive dismissal. Only suspend when you genuinely need to, and keep it as short as possible.
Not every investigation needs an external investigator. For straightforward matters, an internal manager or HR professional can often do the job well. But there are situations where independent investigation makes sense.
At Darwin Gray, our employment lawyers act as independent investigators for organisations across Wales and England, bringing legal expertise and investigative experience to complex workplace situations.
Starting with a conclusion and looking for evidence to support it isn’t investigation, it’s confirmation bias. Keep an open mind until you’ve gathered and analysed all the evidence.
Tribunals regularly criticise employers for superficial investigations. Interview all relevant witnesses, gather all relevant documents, and follow up on leads that emerge during the process.
While thoroughness matters, so does timeliness. Investigations that drag on for months without good reason are unfair to everyone involved and can undermine the process.
In misconduct cases, having the same person investigate and decide on an outcome creates a perception of unfairness. Use different people for each role wherever practicable.
Making decisions without giving the accused employee a chance to give their version of events isn’t just unfair, it can be a fundamental breach of natural justice.
If it’s not documented, it didn’t happen. Keep detailed records of everything: interviews, evidence gathered, decisions made, and reasons for them.
Loose talk about ongoing investigations damages trust, prejudices outcomes, and can lead to claims in its own right.
An investigation that only considers evidence of guilt isn’t fair. You must also look at evidence that supports the employee’s account.
An investigation report should typically include: background on how the investigation arose; the terms of reference; a summary of the process followed (who was interviewed, what documents were reviewed); a summary of the evidence gathered; analysis of the evidence; factual findings; and if appropriate, a recommendation on whether there’s a case to answer.
There’s no statutory right to be accompanied at investigation meetings, unlike disciplinary or grievance hearings. However, many employers allow it as a matter of good practice, particularly for serious allegations. You should follow your own policy, and consider whether allowing a companion would help the meeting run more smoothly.
There’s no fixed timeframe. The investigation should be completed without unreasonable delay, but also be thorough enough to establish the facts. Simple matters might take a few days; complex cases with multiple witnesses could take several weeks. Keep everyone informed of progress and explain any delays.
You can’t force someone to participate, but employees have a duty to cooperate with reasonable management instructions. Refusing to attend investigation meetings without good reason could itself be a disciplinary matter. If an employee won’t participate, proceed with the investigation based on the evidence you can gather.
The civil standard: the balance of probabilities. You’re asking what is more likely than not to have happened, not whether something is proved beyond reasonable doubt.
Recording can create an accurate record but may inhibit open discussion. If you do record, tell participants in advance and provide copies. Many investigators prefer to take detailed notes instead, which can then be shared with the interviewee for confirmation.
If significant new issues come to light, you may need to expand the scope of your investigation or start a separate investigation. Make sure the employee is informed of any new allegations and has the opportunity to respond.
Yes, but be careful not to interfere with the police investigation. You may need to delay some aspects, but you don’t have to put everything on hold. The standard of proof is different (balance of probabilities vs. beyond reasonable doubt), so your conclusions may differ from any criminal outcome.
Rachel Ford-Evans is a Partner in Darwin Gray’s employment team, advising employers on workplace investigations, disciplinary procedures, and employee relations.
With 10 years’ experience in employment law, she regularly conducts complex workplace investigations on behalf of employers, and also advises employers which are conducting internal disciplinary and grievance procedures.
Contact: rford-evans@darwingray.com | 029 2082 9120
Workplace investigations are high-stakes exercises. Get them wrong, and you face unfair dismissal claims, discrimination complaints, and lasting damage to employee relations. Get them right, and you can resolve workplace issues fairly and defensibly.
Independent investigations – We act as external investigators for complex, sensitive, or high-profile matters. Our employment lawyers bring legal expertise to the investigative process, ensuring your investigation will stand up to scrutiny.
Investigation support – If you’re conducting investigations internally, we can advise on process, review terms of reference, and help you avoid common pitfalls.
Investigation training – We train HR teams and managers in conducting fair, effective workplace investigations, covering everything from planning to report-writing.
Post-investigation advice – Once your investigation is complete, we can advise on next steps, whether that’s disciplinary proceedings, grievance outcomes, or wider organisational action.
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