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 and off work, your employer can ask you to give them some form of medical evidence to support payment of SSP (statutory sick pay) or proof of illness, most employers will ask you to obtain a Fit note from your GP.
However, this will also depend on your employer's company 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.
To obtain an initial fit note you will require a consultation with a clinician to discuss the problem that you have been off work with, this can only be done if you have completed a self certification to stand the first 7 days of work.
Continuation fit note
To obtain a continuing fit note, meaning you have had your initial fit note but require further time off work, you can request this via the patient triage (section 2 to the right of this page).
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)