Sick Leave in the UK: Your Complete Guide to Medical Certificates

Let’s start with the rule everyone half-knows but frequently gets wrong.

In the UK, you do not need a doctor’s certificate for the first seven days of illness. You can self-certify — meaning you confirm to your employer, in writing or via a form, that you were genuinely unwell. Many employers have their own self-certification forms. If yours does not, the SC2 form from HMRC covers it.

Here is the part people often get wrong: the seven days include weekends and bank holidays, not just working days. If you fall ill on a Thursday, Saturday and Sunday count toward your seven days. The clock starts from the first day you were sick — not the first day you missed work.

After seven consecutive days of absence, your employer can — and usually will — ask for a fit note from a doctor. At that point, you need to contact your GP, an online GP service, or in some cases a nurse practitioner or physician associate, all of whom are now authorised to issue fit notes.

One more thing worth knowing: your employer’s own sickness policy may be stricter than the legal baseline. Some workplaces require a note after three days, or even for a single day of absence during a certain period. That is legal, provided it is written into their policy — though they cannot specifically demand an NHS fit note for absences under seven days, and may have to cover the cost of any private certificate they require.

The formal name for a sick note in the UK is a Statement of Fitness for Work, more commonly called a fit note or Med3. It replaced the old-style sick note back in 2010, and the change was more significant than most people realise.

Under the old system, a doctor either signed you off or they did not. The fit note introduced a third option: “may be fit for work.” This means your GP can suggest a phased return, reduced hours, amended duties, or workplace adaptations — rather than simply keeping you off entirely. If your employer can accommodate those adjustments, you can return to work. If they cannot, the note effectively functions as a full sign-off and SSP continues.

A fit note does not need to state your diagnosis in detail. It confirms that you are either not fit for work or may be fit with adjustments, and it states the recommended time period. That is all your employer is legally entitled to know. Your specific condition is private medical information.

One distinction that trips people up: a standard NHS fit note (Med3) and a private medical certificate are not identical in legal terms. Both are accepted by employers for workplace purposes. However, if you are applying for Employment and Support Allowance, Universal Credit, or any other state benefit that requires medical evidence, you need an NHS fit note from a registered GP — private certificates are not accepted for benefit claims. This is important to know before you opt for a quick online private note.

Stress, anxiety, burnout, and depression now account for a significant and growing share of sick leave in the UK — yet many employees still do not know their full rights in this area.

A fit note for a mental health condition works exactly the same way as one for a physical illness. Your GP does not need to write your diagnosis in clinical detail. The note confirms unfitness for work and recommends a timeframe. You are under no legal obligation to tell your employer the specific nature of your condition — only that you have a medical reason for absence.

Under the Equality Act 2010, long-term mental health conditions that substantially affect day-to-day life may qualify as disabilities, which means your employer has a legal duty to make reasonable adjustments. This could include a phased return, flexible hours, or a temporary change in responsibilities.

If you are struggling and your doctor recommends time off, ask directly for a fit note at the appointment. GPs are generally supportive, and you are entitled to that documentation.

This is where a lot of published guidance is now out of date, so pay attention.

The Employment Rights Act 2025 made significant changes to Statutory Sick Pay (SSP) that came into effect on 6 April 2026. Here is what changed:

SSP is now payable from day one of sickness absence. The previous system made employees wait three unpaid “qualifying days” before SSP began. Those waiting days no longer exist.

The lower earnings limit has been scrapped. Previously, you needed to earn at least £125 per week on average to qualify for SSP. That threshold is gone. If you work and pay tax through PAYE — including agency workers and many zero-hours staff — you are now eligible from your first day of work, provided you have actually started working.

The rate has also changed. For lower earners, SSP is now calculated at 80% of average weekly earnings rather than the flat weekly rate, whichever figure is lower. Higher earners still receive the standard flat rate. SSP is paid for a maximum of 28 weeks per period of illness.

To put this in practical terms: if you earn £200 per week and are off sick, you would receive £160 per week under the 80% calculation. If you earn £500 per week, you receive the standard flat rate (£118.75 for 2025/26, updated annually). Your employer pays SSP through payroll and deducts tax and National Insurance in the usual way.

One thing that has not changed: SSP is the legal minimum. Many employers offer contractual or occupational sick pay on top of this, which may be full pay for a set number of weeks. Check your employment contract — it will be written in there if it applies to you.

This is a real and frustrating problem across the UK right now, and no one seems to address it directly. NHS waiting times mean that getting a GP appointment within a reasonable timeframe is not always possible — but there are options.

Nurse practitioners and physician associates are now fully authorised to issue fit notes. If your GP practice offers appointments with either, you do not need to wait for a doctor specifically.

Online GP services — provided they are staffed by GMC-registered doctors — issue legally valid fit notes, typically within 24 hours. These are accepted by employers in the same way as an in-person GP note.

If you were admitted to hospital during your illness, a discharge letter can often serve as supporting evidence of your absence, and some employers will accept it in place of a fit note.

If your employer is pushing for a note urgently and you genuinely cannot obtain one due to NHS access issues, put that in writing. Document your attempts to contact your GP. ACAS guidance makes clear that employees should not be penalised for circumstances outside their control.

Technically, no — not if it is valid. UK employers are legally required to accept a fit note from any GMC-registered doctor, whether NHS or private. If your employer refuses to accept a legitimate fit note without good reason, that may amount to unfair treatment under the Employment Rights Act 1996.

That said, employers can ask for a second opinion or request further information if they have genuine grounds to doubt the certificate. They should not do this routinely or as a means of pressure.

If a dispute arises, ACAS offers free guidance and conciliation. Unresolved disputes can be taken to an Employment Tribunal, with claims usually needing to be brought within three months less one day of the act complained of.

Understanding when and how to use a medical certificate for sick leave in the UK puts you in a much stronger position — whether you are an employee navigating illness, an HR professional handling absence, or a manager trying to apply the rules fairly. The April 2026 SSP changes have shifted the landscape significantly, making sick pay more accessible than it has been in years.

Know your rights, get the right documentation, and do not let uncertainty about the process make a difficult time harder than it needs to be.