Gathering evidence: what does a ‘reasonable’ workplace investigation require?

A version of this article was first published in People Management in September 2025.

The investigation is a key element of any disciplinary or grievance process so, in the event of a claim, an employment tribunal will look closely at how the employer has carried it out. The Acas code of practice on disciplinary and grievance procedures requires employers to carry out ‘necessary investigations’ to establish the facts of a case. In addition, the accompanying Acas guidance provides that investigations must be ‘reasonable’, a standard also applied by the tribunal. But what is a ‘reasonable investigation’?

What is reasonable depends on the specific circumstances, but in essence the scope of the investigation needs to be proportionate to the issues under investigation. Importantly, an investigator needs to make a decision as to whether something did or did not happen on ‘the balance of probabilities’, which, importantly, is a lower burden of proof than the criminal standard of ‘beyond reasonable doubt’.

When determining whether an investigation is reasonable, a tribunal will consider whether it was within the range of investigations that would be carried out by a reasonable employer. The complexity and seriousness of the issues, the potential impact on the employee, the employer’s size and resources and any potential regulatory elements are all factors a tribunal will consider.

There is no ‘one-size-fits-all’ approach, but the evidence collected in any investigation will pay a key role in determining the reasonableness (or otherwise) of that investigation. The investigator should gather enough relevant evidence to support a reasonable belief in the facts, keeping in mind the scope of the investigation and the issues to be decided.

Sources of evidence

At the outset of an investigation, the investigator should identify potential sources of evidence relating to each issue under investigation. Gathering evidence can be a significant undertaking so keep the scope of the investigation firmly in mind to ensure the investigation remains proportionate and reasonable in the circumstances. When gathering evidence the investigator should consider the feasibility of obtaining evidence, including any data protection and regulatory considerations.

Evidence may include:

• Witness evidence
• Written evidence, including emails, meeting notes, handwritten notes and instant messages, including Teams or WhatsApp.
• CCTV footage
• Medical records

The investigator should also consider how best to secure the evidence so that confidentiality is maintained. Information should be shared on a ‘need to know’ basis and evidence secured, ideally with passwords or encryption. Everyone involved in the investigation should also be reminded to keep details of the investigation confidential.

Selecting witnesses and preparing for interviews

People are often the most valuable source of evidence – carefully consider who to speak to and the order in which to interview them, bearing in mind the chronology of events and witness availability.

The employee at the centre of the issue will be a key witness. Others may be direct observers or may be able to provide relevant context but always keep in mind the investigation must remain proportionate. For example, if 30 people witness an alleged incident, it is unlikely to be proportionate to interview all 30. Focus on the key witnesses, such as those closest to the event.

The investigator should prepare questions for each witness in advance but it is important that they remain flexible and are prepared to deviate to address unexpected issues that may arise. Keep questions open to encourage full accounts, and be ready to probe if other evidence contradicts a statement. It is generally preferable for interviews to take place in person, but virtual interviews may be used if needed. Afterwards, the witness should be asked to confirm in writing that they agree with the statement or notes taken during the interview. Sometimes a witness may suggest another person to interview or refer to additional evidence. Where proportionate, the investigator should follow up by speaking to further witnesses or reviewing new documents as needed.

Unless your policies say otherwise, employees don’t have the right to be accompanied at an investigation meeting (although take care to ensure that meetings don’t morph into a grievance or disciplinary meeting, which would trigger the right to bring a companion), but consider whether it is appropriate to make an exception, for example, as a reasonable adjustment for employees with disabilities, or if a witness is young, say a new apprentice.

Being interviewed as a witness may be daunting and supporting witnesses ahead of and during the interview will help the process run more smoothly. Choose interview locations that offer privacy, especially for sensitive matters, where off-site venues may be preferable. Particular care should be taken where the alleged conduct, such as sexual harassment, may have had a traumatic effect on the complainant.

Some witnesses ask for anonymity. This poses a challenge, particularly in a disciplinary investigation as the employee facing action has the right to know the case against them, which will usually include knowing the identity of any witnesses. In these circumstances, take the time to understand, and where possible, address the witness’ concerns. However, never promise total anonymity as it would be very difficult to preserve anonymity if the matter went to an employment tribunal.

Proportionality is key

When carrying out an investigation and considering the extent of the evidence required, the investigator should keep in mind the nature and scope of the investigation and what is reasonable in the circumstances. This won’t always involve turning over every stone, and investigators should remember the importance of balancing thoroughness with proportionality.