Been off work?
You do not require a doctor's sickness certificate for any illness lasting seven days or less. Your employer may however require you to complete a self-certification form (employee's statement of sickness) which is available from your employer or you can download the form from the HMRC website.
Evidence that you are sick
If you are sick for more than seven days, your employer can ask you to give them some form of medical evidence to support payment of statutory sick pay.
It is up to your employer to decide whether you are incapable of work. A medical certificate, now called a 'Statement of Fitness for Work’ (see below) from your doctor is a strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherwise.
Statement of Fitness for Work - 'Fit Note'
The 'fit note' was introduced on 6 April 2010. With your employer's support, the note will help you return to work sooner by providing more information about the effects of your illness or injury.
You can find more information about Statutory Sick Pay (SSP) at the HMRC website.