You do not require a doctors sickness certificate if you have had any illness for 4-7 days in a row, you can self–certify your illness using a SC2 form. You can obtain this form from your employer or from on the HMRC website.
Statement of Fitness for Work
The Doctor will provide you with a Statement of Fitness for Work (‘Fit Note’) if you are still not well enough to work. Your employer will most likely request this statement as evidence to support payment of Statutory Sick Pay (SSP).
Further information is available at the Direct.Gov website. 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.
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 (where this information was sourced)
If you or your employer require further information concerning your period(s) of illness, then your Doctor may require to issue you with a letter. Please note that that there may be a charge for this service.