- An investigation establishes facts, it does not decide outcomes. It should be run by someone independent of the events, against clear terms of reference, with findings reached on the balance of probabilities.
- The Acas Code of Practice is the benchmark. Employers are expected to investigate without unreasonable delay, and tribunals can adjust awards by up to 25% where the Code has been unreasonably ignored.
- The stakes rise from January 2027. Under the Employment Rights Act 2025, the qualifying period for ordinary unfair dismissal falls from two years to six months and the compensation cap is removed.
Why the investigation matters more than the hearing
When an employment dispute reaches a tribunal, it is rarely the disciplinary hearing that loses the case for the employer, but the investigation that came before it. A rushed round of interviews, a manager who had already made up their mind, or a report that quietly passed over the evidence that did not fit are the details that turn a defensible decision into an unfair one.
The legal test in conduct cases has been settled for decades. Following British Home Stores v Burchell, an employer needs a genuine belief in the misconduct, reasonable grounds for that belief, and an investigation that was as thorough as the circumstances required, and the Court of Appeal later confirmed in Sainsbury’s v Hitt that the investigation itself is judged against the range of responses open to a reasonable employer.
That bar is about to apply far more widely. From 1 January 2027, employees will need only six months’ service to bring an ordinary unfair dismissal claim and there will be no cap on compensation, so investigations involving newer staff, which some organisations have historically treated more lightly, will need to stand up to the same scrutiny as any other.
Step 1: Decide whether a formal investigation is needed
Not every concern needs a formal process, and starting one can harden positions and make a resolvable situation considerably worse. Where the real issue is a breakdown in a working relationship, an early conversation or workplace mediation will often achieve more than an investigation ever could.
A formal investigation is usually the right route where the allegation could lead to disciplinary action, where someone has raised a formal grievance, or where there are allegations of bullying, harassment or discrimination. If it is unclear what actually needs to be established, a short preliminary fact-finding exercise can help before committing to a full process.
Step 2: Choose the right investigator
The investigator’s credibility underpins everything that follows. Acas guidance says they should, as far as possible, be unconnected with the matters under investigation, which in practice often rules out the employee’s line manager and anyone who witnessed the events or has a stake in the outcome.
The Acas Code of Practice also says that, where practicable, different people should carry out the investigation and the disciplinary hearing. Smaller organisations frequently cannot manage this, and even larger ones can struggle when allegations involve senior leaders, which is why many choose to bring in an external investigator or to have an experienced professional mentor and supervise their own internal investigator through the process.
More on how CMP, the UK’s largest provider, carries out an investigation here.
Step 3: Set clear terms of reference
Terms of reference set out the investigator’s remit in writing, which prevents scope creep, protects the investigator from accusations of overreach, and gives the eventual decision-maker a clear framework. They should cover:
- the specific allegations or complaints being investigated
- who has commissioned the investigation and who will receive the report
- whether the investigator should make findings only, or recommendations as well
- the policies that apply and an expected timescale
If new issues emerge along the way, the investigator should return to whoever commissioned the work rather than quietly widening the scope.
Step 4: Plan before interviewing anyone
Acas recommends a short investigation plan listing who needs to be spoken to and in what order, which documents or records to collect, and the main questions each interview needs to answer. Evidence goes well beyond witness accounts, and emails, messages, rotas, CCTV and access logs should be secured early before anything is deleted or overwritten.
This is also the point to consider suspension, which should never be automatic. It is meant to be a neutral act to protect the investigation or the people involved, should be as short as possible and normally on full pay, and can itself damage trust and confidence if used without good reason.
Step 5: Run fair, well-recorded interviews
Interviews are where most investigations succeed or fail, and a handful of principles make a significant difference.
Give people proper notice
Each person should know why they are being invited, who will be present and roughly how long it will take, and the person under investigation should understand the allegations well enough to respond to them properly.
Be clear about accompaniment
There is no statutory right to be accompanied at an investigation meeting, since that right applies to formal disciplinary and grievance hearings, but allowing a companion is often sensible where someone is anxious or vulnerable, and your own policy may already permit it.
Ask open questions and follow the leads
Start broad and narrow down, avoid leading questions, and give the person under investigation a genuine chance to put their side and name any witnesses or evidence they think is relevant, then follow those up.
Keep accurate records
Notes should be reviewed and signed by the interviewee, bearing in mind they may later be disclosed at a tribunal or through a subject access request. Witnesses can be protected where there is good reason, but they should not be promised total confidentiality, because the person under investigation is usually entitled to know the substance of the evidence against them.
Step 6: Weigh the evidence and write the report
The investigator’s job is to assess the evidence, not simply to list it, which means asking for each allegation what is agreed, what is disputed, and which account is better supported by the evidence as a whole. The standard is the balance of probabilities, and the investigator must look as carefully for evidence that supports the employee as for evidence against them, particularly where their career or reputation is at stake.
Acas provides a free report template, and a good report typically sets out the terms of reference, the method used, a summary of the evidence and the investigator’s findings on each allegation with reasons, together with recommendations where the terms of reference ask for them. The conclusions must be the investigator’s own, because HR can advise on process and structure but tribunals take a dim view of reports edited to fit a preferred outcome.
Step 7: Close the process properly
Once the report is submitted, the decision about what happens next belongs to someone else, but two things are easily forgotten. Everyone involved, including witnesses and whoever raised the complaint, should be told the investigation has concluded, and some thought should go into the working relationships left behind, since people will often have to keep working together whatever the outcome, and a facilitated conversation or mediation can make that considerably easier.
How CMP Solutions can help
Many organisations run investigations well internally, but external support makes sense where no one is sufficiently independent, where allegations involve senior leaders or whistleblowing concerns, or where the case simply needs to be handled promptly without pulling managers away from their day jobs. An independent investigator also brings something that is hard to achieve internally, which is the perception of fairness, and employees who trust a process are far more likely to accept its outcome.
CMP Solutions has a team of over 100 investigators who carry out grievance and disciplinary investigations for employers, regulators and public bodies across the UK, and we can also act as hearing manager, panel chair or appeal manager. For organisations with a steady volume of cases, we can take on grievance handling as an outsourced service, and our training and development programmes help build investigation skills within your own team.
If you are facing a complaint and want to get the process right from the start, get in touch with our team to talk it through.
