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’ (see below) from your doctor is evidence that you are ill and unable to work for a set period of time or in some cases fit to return to work with amended duties, altered hours, a phased returned to work or work place adaptations.
Dentist and hospital Doctors can also supply you with medical certificates for your employer.
For more information on FIT NOTES see the DirectGov website (where this information was sourced).