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 consecutive days (including non-working 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'
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.
The fit note will say the employee is either ‘not fit for work’ or ‘may be fit for work’. If it says the employee ‘may be fit for work’, employers should discuss any changes that might help the employee return to work (for example, different hours or tasks). The employee must be treated as ‘not fit for work’ if there’s no agreement on these changes.
Employers can take a copy of the fit note. The employee should keep the original.
Please note that a 'Fit Note' will not be issued until seven (7) days absence. The patient should 'self-certify' the first seven days.
If you (or your employer) require a fit note before the seven days have elapsed, this will be treated and changed as a non-NHS service (currently £60 + VAT)
For more information see the DirectGov website (where this information was sourced) and from 'Taking Sick Leave'.