You do not require a doctor’s sickness certificate for any illness lasting seven days or less. Your employer can ask you to confirm that you’ve been ill. You can do this by filling in a form yourself when you return to work. This is called self-certification. A self-certification form (SC2) is available from your employer or on the HMRC website.
Medical evidence that you are sick
If you’re sick and off work for more than seven days your employer will probably ask for proof of your illness. Most employers ask for a 'fit note' from your GP. We can only provide a note if we have seen you with the medical problem causing the absence or have received a communication about the problem from another health professional - such as A&E or the out of hours service.
You could also provide evidence from someone who is not a GP. For example if you have been admitted to hospital the ward staff will provide a 'fit note'.
Statement of Fitness for Work – ‘Fit Note’
The ‘fit note’ was introduced on 6 April 2010. With your employer’s support, the note can help you return to work sooner by providing more information about the effects of your illness or injury.
For more information see the DirectGov website (where this information was sourced)