Sickness lasting less than one week
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 you can download a copy from here Self_Certificate.pdf ,completed by you and sent to your employer.
Sickness lasting more than one week
If you are sick for more than seven days, your employer can ask you to provide some form of medical evidence to support payment of SSP (statutory sick pay). For this, a "Fit Note" (see below) can be provided by your GP
The "Fit Note" was introduced on 6 April 2010 to take the place of the old "Sick note" The "Fit note" is provided by your doctor after you have been off sick for more than seven days. It states that the doctor has reviewed your condition and and suggests what can be done by your employer, to help you back to work. 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)