Doctor's Sickness Certificate
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 (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 days, your employer can ask you to give them some form of medical evidence to support payment of SSP (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 strong evidence that you are sick 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.
Statement of Fitness for Work - "Fit Note"
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.
Please note: you should allow 5 working days for your fit note to be ready for collection. Fit notes can be back-dated by your GP and even if a previous note has expired, will not be treated as urgent but will require a routine telephone call with your GP.
For more information see the DirectGov website (where this information was sourced)