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 Gov.uk website.