When Is an Employer's Investigation Considered Unreasonable?

In UK unfair dismissal law, the quality of an employer's investigation is a central factor in determining whether a dismissal is fair. Even where misconduct or capability concerns exist, an Employment Tribunal will examine whether the employer carried out a reasonable investigation before making a decision.

An investigation does not need to be perfect, but it must fall within the range of reasonable responses expected of a reasonable employer. If the investigation is rushed, incomplete, biased, or unfair, the dismissal may be found unfair under the Employment Rights Act 1996.

Legal Framework for Workplace Investigations

Under section 98(4) of the Employment Rights Act 1996, fairness depends on whether the employer acted reasonably in all the circumstances, including:

• The reason for dismissal
• The size and resources of the employer
• The disciplinary procedure followed
• The quality of the investigation

The ACAS Code of Practice on Disciplinary and Grievance Procedures provides the main practical benchmark for what constitutes a fair investigation.

Employment Tribunals take the ACAS Code into account and may increase or reduce compensation where it is not followed.

What Is a Reasonable Investigation?

A reasonable investigation is one that allows the employer to establish the facts fairly before making a decision. It typically includes:

• Identifying the allegations clearly
• Gathering relevant evidence
• Interviewing witnesses where appropriate
• Reviewing documents, emails, or records
• Allowing the employee to respond to the allegations
• Ensuring impartiality of the investigating officer

The key requirement is that the employer must carry out a reasonable inquiry in all the circumstances, not an exhaustive or legal-level investigation.

When an Investigation Becomes Unreasonable

An investigation may be considered unreasonable where it falls below expected standards of fairness or completeness.

1. Failure to Investigate Key Evidence

An investigation is often found unreasonable if the employer:

• Ignores relevant documents or communications
• Fails to interview key witnesses
• Does not review CCTV or digital evidence when available
• Relies on assumptions rather than facts

Tribunals expect employers to make reasonable efforts to obtain the best available evidence before deciding on dismissal.

2. Biased or Prejudged Investigation

An investigation must be impartial. It becomes unreasonable where:

• The investigator has a conflict of interest
• The outcome appears predetermined
• Evidence is selectively considered
• Only evidence supporting dismissal is gathered

A lack of independence can significantly undermine fairness.

3. Failure to Put Allegations to the Employee

A fundamental requirement of procedural fairness is that the employee must be:

• Told what they are accused of
• Given an opportunity to respond
• Allowed to explain or challenge evidence

An investigation is likely to be unfair if the employee is not properly informed or consulted before conclusions are reached.

4. Rushed or Superficial Investigation

Investigations may be unreasonable where they are:

• Completed too quickly without proper inquiry
• Based on limited or surface-level checks
• Conducted without sufficient detail
• Driven by a desire to reach a quick outcome

Speed alone is not the issue; the problem is lack of thoroughness.

5. Failure to Follow Internal Procedures or ACAS Guidance

Employers are expected to follow:

• Their own disciplinary or grievance policies
• The ACAS Code of Practice

Failure may include:

• Skipping formal steps
• Not holding an investigation meeting
• Not issuing investigation reports
• Failing to allow accompaniment where required

Deviations from procedure must be justified to be considered reasonable.

6. Inconsistent Treatment of Evidence

An investigation may be unreasonable if:

• Some witnesses are interviewed but others are ignored without reason
• Exculpatory evidence is dismissed without explanation
• Different standards are applied to different employees in similar situations

Consistency and fairness in evidence handling are key tribunal considerations.

7. Overreliance on One Source of Evidence

It may be unreasonable to base conclusions solely on:

• A single complaint
• One witness statement without corroboration
• Unverified allegations
• Subjective opinion without factual support

Employers are expected to weigh evidence critically, not accept it unchallenged.

8. Failure to Consider the Employee's Explanation

A proper investigation must consider the employee's version of events. It is often unreasonable where:

• The employee's account is not recorded or assessed
• Explanations are dismissed without evaluation
• No opportunity is given to clarify inconsistencies

Tribunals often criticise employers who fail to engage meaningfully with the employee's defence.

9. Inadequate Record-Keeping

Poor documentation can undermine the fairness of an investigation. Issues include:

• No written investigation report
• Missing notes from interviews
• Lack of evidence trail for decision-making
• Unclear reasoning for conclusions

While minor administrative issues may not be fatal, significant gaps can render the process unreasonable.

10. Investigation Conducted by the Decision-Maker Alone

Fairness is compromised where the same individual:

• Investigates the allegation
• Decides the outcome
• Conducts the disciplinary hearing

Separation between investigation and decision-making is generally expected to ensure impartiality.

How Employment Tribunals Assess Investigations

Tribunals do not require perfection. Instead, they assess whether the investigation fell within the band of reasonable responses open to a reasonable employer.

Key questions include:

• Was the investigation sufficient to form a fair view of the facts?
• Were relevant lines of inquiry pursued?
• Was the employee given a fair opportunity to respond?
• Was the process impartial and evidence-based?

The size and resources of the employer are also taken into account.

Consequences of an Unreasonable Investigation

If an investigation is found unreasonable, potential outcomes include:

• A finding of unfair dismissal
• Compensation awards (basic and compensatory)
• Possible reduction or increase in compensation depending on ACAS Code compliance
• Reinstatement or re-engagement in rare cases

Even if misconduct is proven, procedural unfairness alone can make a dismissal unlawful.

Time Limits for Bringing a Claim

An unfair dismissal claim must usually be brought within:

• Three months less one day from the effective date of termination

Before filing a claim, the employee must complete ACAS Early Conciliation, which may pause limitation periods.

Common Workplace Scenarios

Unreasonable investigations often arise in cases involving:

• Allegations of misconduct without proper evidence review
• Disciplinary action following workplace complaints or grievances
• Performance concerns escalated without documentation
• Conflicts between employees where only one version is considered
• Rapid dismissals following isolated incidents

These cases frequently turn on procedural fairness rather than the underlying allegation.

Key Principles

A fair investigation does not require legal precision or exhaustive inquiry. However, it must be:

• Impartial
• Evidence-based
• Proportionate to the seriousness of the allegation
• Capable of supporting a fair decision

Where these standards are not met, the investigation may be considered unreasonable, leading to an unfair dismissal finding.

Key Takeaways

An employer's investigation becomes unreasonable when it fails to meet basic standards of fairness, impartiality, and thoroughness. Common issues include ignoring key evidence, failing to interview witnesses, bias, rushed processes, and lack of engagement with the employee's explanation.

Employment Tribunals assess whether the investigation was within the range of reasonable responses, not whether it was perfect. However, significant procedural failings can render a dismissal unfair even if misconduct or performance concerns exist.