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).
If you require a fit note from your GP, contact the surgery to book a face to face appointment. If an extension to this fit note is required, please contact the surgery, prior to your fit note expiring.
Statement of Fitness for Work - ’Fit Note'
The 'fit note' was introduced on. 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).