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.
Initial Statement of Fitness for Work
You will need to make a routine appointment with a GP if your employer has requested a statement as evidence that you are unable to work. The GP will assess you and if they feel that you qualify for a statement you will be provided with one to pass onto your employer.
This statement will also have a date that you are expected to be fit to return to work.
Continuation of Statement of Fitness for Work
If you still feel you are unable to return to work when your statement will expire, you will need to request a continuation the day before.
This will usually take 48 hours to process and can only be completed by the GP who assessed you and signed your initial statement.
Please be aware that there are incidences where this may take longer due to GP's being out of the surgery.
For more information see the DirectGov website (where this information was sourced)