Illness lasting seven days or less: you do not require a doctor’s sickness certificate. Your employer may however require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website.
Illness lasting more than seven days: your employer may ask you to provide form of medical evidence to support payment of SSP (statutory sick pay). A medical certificate, now called a ‘Statement of Fitness for Work’ (‘fit note’) from your doctor would normally be accepted.
For more information see the DirectGov website