You do not require a doctor's sickness certificate for any period of illness lasting up to seven days. Your employer may however require you to complete a self-certification form (SC2), which is available from your employer, the surgery reception desk or on the HMRC website.
If you're sick and off work for more than seven days, your employer may ask for proof of your illness. Most employers ask for a statement of fitness to work (fit note) from your GP.
However, this will also depend on your employer's policy on sick leave (or sickness absence). This policy should tell you how many days you can be off sick before you need to provide proof of illness or a fit note.
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 have been in hospital or have been advised to refrain from work by a hospital consultant or specialist, they will provide a medical certificate if it is required.
Your GP can only provide sickness certification if they have assessed you and they are satisfied that you are unfit for work. In order to do this they will need to have seen or spoken to you at the time that you were ill or have had sight of a hospital letter or a report from another doctor concerning your illness. Your GP cannot provide retrospective certification for a spell of sickness if they do not have any clear evidence that you were unfit for work at that time.
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 (where this information was sourced)