Your employer cannot ask you for the first 7 days that you are sick (HMRC statutory law)
Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website.
Evidence that you are sick
If you are sick for more than seven (calendar) days, your employer may ask you to give them some form of medical evidence to support payment of SSP (statutory sick pay). Before that time, you will have to fill in a self-certificate.
A medical certificate, now called a 'Statement of Fitness for Work’ (see below), from your doctor is strong evidence that you are unwell and would normally be accepted, unless there is evidence to prove otherwise.
You could also provide evidence from someone who is not a medical practitioner, e.g. a dentist. Your employer will decide whether or not this evidence is acceptable. If your employer has any doubts, they may still ask for a medical certificate from your GP.
If you are admitted to an hospital, the doctors who are responsible for you should provide you with a certificate when you are discharged.
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.
For more information see the DirectGov website