A written warning is a formal disciplinary sanction. It records that your employer believes your conduct or performance has fallen short, and it sets a period in which you must put that right. It does not end your job, but it changes your position: while the warning is live, a further problem can be treated more seriously, and a final written warning brings dismissal into view. Most employees want to know one thing first — how long the warning stays on their record. This guide answers that, and explains what your employer must do before a warning is issued, when a warning is unfair, and how to challenge one.
Contents
- What a written warning is
- The stages of a disciplinary warning
- How long a written warning lasts
- What must happen before a warning is issued
- What a written warning letter must say
- When a warning is unfair
- How to challenge a written warning
- Warnings and dismissal
- FAQs
- How the National Employees Union can help
What a written warning is
A written warning is a sanction imposed at the end of a disciplinary process. Your employer uses it to record a finding of misconduct or poor performance, to tell you what must change, and to warn you what happens if it does not.
The law does not set out a fixed system of warnings. There is no statute that says you must receive one warning, or two, before you can be dismissed. Instead the standard comes from the ACAS Code of Practice on disciplinary and grievance procedures, which an employment tribunal must take into account in any relevant case. An unreasonable failure to follow the Code can increase or reduce a tribunal award by up to 25%.
That is why the Code matters so much in practice. It sets the minimum your employer should do, and most workplace disciplinary policies are built on top of it. Your own employer’s policy is the second document to read, because it may give you more than the Code does — and where it does, your employer should follow it.
The stages of a disciplinary warning
Most disciplinary procedures move through a sequence of sanctions, escalating only if the problem continues.
| Stage | Typical use | Usual live period |
|---|---|---|
| Informal word or verbal warning | Minor, first-time issues | 6 months |
| First written warning | Misconduct, or a repeat after informal action | 6 to 12 months |
| Final written warning | Repeat misconduct, or serious first offence | 12 months |
| Dismissal or other sanction | Further misconduct while a final warning is live, or gross misconduct | Not applicable |
The periods above are custom and practice, not law. Your employer sets them in its own policy, and ACAS guidance offers six months for a first warning and twelve months for a final warning as examples rather than rules.
Your employer can also skip stages. The Code allows a final written warning as a first sanction where the conduct is serious enough, for example where your actions have had, or could have had, a serious impact on the organisation. Skipping straight to dismissal is only justified where the conduct amounts to gross misconduct.
How long a written warning lasts
A written warning lasts for the period your employer specifies in the warning letter. That period is often six or twelve months, and it should be proportionate to what happened.
When the period expires, the warning “expires” or becomes “spent”. It should then be disregarded for disciplinary purposes. If you commit further misconduct after that date, your employer should normally treat it as a first offence and start again at the bottom of the procedure.
The record itself does not have to be destroyed. Your employer can keep the paperwork on your personnel file for as long as it has a lawful reason to, under UK GDPR and the Data Protection Act 2018. The distinction is between keeping a record and relying on it. A spent warning should not be used to justify a heavier sanction later.
Some employers issue warnings with no expiry date. A permanent warning is very difficult to justify, and a warning without a timescale gives you nothing to work towards. Raise it on appeal.
What must happen before a warning is issued
A warning is only as sound as the process behind it. The ACAS Code sets out the steps your employer should follow.
A reasonable investigation
Your employer should establish the facts before deciding anything. That usually means an investigation meeting, documents, and witness evidence. An investigation is fact-finding, not judgment, and the investigator should ideally not be the decision-maker.
Written notice of the allegations
You should be told the allegations in writing, with enough detail and evidence to prepare a response, and given reasonable notice of the disciplinary meeting.
A hearing
You should have a proper opportunity to put your case at a disciplinary hearing at work before any sanction is decided. A meeting where the decision has already been made is not a hearing.
The right to be accompanied
Under section 10 of the Employment Relations Act 1999, you can be accompanied at a disciplinary hearing by a trade union representative or a work colleague. Your companion can put your case, sum up, and confer with you.
A decision and a right of appeal
The outcome should be confirmed in writing, and you must be told you can appeal. Each of these steps is a place where an employer can go wrong, and each failure weakens the warning that follows.
What a written warning letter must say
A properly written warning letter is specific. It should set out the nature of the misconduct or poor performance, the improvement required, the timescale for that improvement, how long the warning will remain live, the consequences of further misconduct or failure to improve, and your right of appeal.
If the letter is vague about what you did or what you must change, you cannot comply with it. That is a fair point to make in an appeal. Where the concern is performance rather than conduct, your employer may also set improvement objectives, and those must be clear and measurable too.
When a warning is unfair
A written warning can be challenged on substance, on process, or on proportionality.
Common grounds include: the conduct did not happen, or not as alleged; there was no reasonable investigation; you were not told the allegations before the meeting; you were refused a companion; the decision-maker had already made up their mind; the sanction is out of proportion to the conduct; other employees were treated more leniently for the same thing; or the real reason for the warning is something else entirely.
That last point matters. A warning issued because you raised a grievance, reported wrongdoing, took part in trade union activities, requested flexible working, or took sickness or family leave is not a genuine disciplinary sanction. It is a detriment, and separate legal protections apply. Where the underlying dispute has two sides, the wider process is explained in our guide to disciplinary and grievance procedures.
How to challenge a written warning
Appeal. It is the fastest and cheapest route, and it is the one route employers are obliged to offer.
Appeals must normally be submitted in writing within a short deadline — often five working days from the outcome letter. Check your policy and diarise the date. Your appeal should identify what was wrong: the finding, the process, the sanction, or new evidence. Say what outcome you want: the warning removed, reduced, or shortened.
The appeal should be heard by someone who was not involved in the original decision, and you have the same right to be accompanied. Our guide on how to appeal a disciplinary decision sets out how to structure the letter and the hearing.
A written warning on its own is not usually something you can take to an employment tribunal. There is no free-standing claim for “unfair warning”. You can bring a claim where the warning amounts to unlawful discrimination, or a detriment for whistleblowing or trade union activity, or where it forms part of conduct serious enough to found a constructive dismissal claim. Otherwise, the warning matters most later, if it is used to dismiss you.
Warnings and dismissal
When a tribunal looks at a dismissal that followed earlier warnings, it does not usually re-run the earlier disciplinary cases. It asks whether the employer acted reasonably in treating the final incident as sufficient reason to dismiss, under section 98(4) of the Employment Rights Act 1996.
A live final written warning makes dismissal much easier for an employer to justify. That is why challenging a warning at the time is worth the effort, even if the sanction feels survivable. A warning you accept without appeal is harder to attack a year later.
There are limits. An employer cannot rely on a warning that was manifestly inappropriate — one issued in bad faith, or with no proper basis at all. And an expired warning should not be used to tip the balance towards dismissal. If you have been dismissed on the back of warnings you believe were unsound, see our guide to unfair dismissal.
Time limits are strict. To bring most tribunal claims you must start ACAS Early Conciliation within three months less one day of the act you are complaining about, or of the effective date of termination. From 1 October 2026 the time limit for most tribunal claims increases from three months to six months, under the Employment Rights Act 2025. Do not rely on the longer period until it applies to your case.
FAQs
How long does a written warning stay on your record?
A written warning stays live for the period stated in the warning letter, commonly six or twelve months for a first written warning and twelve months for a final written warning. After that it expires and should be disregarded for disciplinary purposes. Your employer can keep the paperwork on file, but should not rely on it to increase a later sanction.
Can I be sacked while I have a written warning?
Yes, but not simply because a warning exists. Dismissal usually requires further misconduct or a failure to improve while the warning is live, or conduct serious enough to amount to gross misconduct on its own. Your employer must still investigate, hold a hearing, and offer an appeal.
Do I have to sign a written warning?
No. Signing is normally only an acknowledgement that you received the letter, not an admission that you agree with it. If you are asked to sign, you can write “received, not agreed” next to your signature, or decline and confirm receipt by email instead.
Can a written warning be removed from my file?
It can be removed or reduced on appeal, or by agreement. Once the live period ends, the warning expires automatically, whether or not the document is deleted. You can make a data subject access request to see what is held about you.
Can my employer extend a written warning?
Only if its disciplinary policy allows it, and even then extending a live warning without a fresh disciplinary process is difficult to justify. If your employer extends a warning because of a new incident, that new incident should go through the full procedure. Raise any extension in an appeal.
Does a written warning show up in a reference?
Most employers now give factual references covering dates and job title only. There is no legal duty to disclose a warning, but a reference must not be misleading. A live warning is more likely to be mentioned than an expired one.
How the National Employees Union can help
A written warning is easier to prevent than to remove. Getting advice before the disciplinary hearing, not after the outcome letter, changes what happens.
National Employees Union members get:
Advice on the allegations and the evidence before you respond
A legally trained rep to accompany you at disciplinary and appeal hearings
Help drafting your written response and appeal letter
A review of your employer’s policy against the ACAS Code
Support if the warning leads to dismissal or a tribunal claim
Deadlines for appeals are short. Join the National Employees Union today
Membership from £15/month.
Written by the National Employees Union team. We represent employees in disciplinary processes across the UK. This guide is reviewed regularly and was last updated in August 2026. For advice on your specific situation, contact us.
