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).
A medical certificate, now called a 'Statement of Fitness for Work’ is available from your GP if required. It is also available from other practitioners such as physiotherapists, hospital doctors and dentists.
Statement of Fitness for Work - ’Fit Note'
The 'fit note' was introduced on 6 April 2010. This may certify you as sick but also certify you as fit to work with your employer's support which may mean fewer hours or different work, on a temproary basis.
For more information see the DirectGov website (where this information was sourced)