Sickness Certificates
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 SSP (statutory sick pay).
It is up to you employer to decide whether you are incapable of work. A medical certificate, now called ‘Statement of Fitness for Work’ (see below) from your doctor is strong evidence that you are sick and would normally be acepted, unless there is evidence to probe otherwise.
You could also provide evidence from someone who is not a medical practitoner, e.g. a dentist. Your employer will decide whether or not this evidence is acceptable. If your employer has any doubts, they may still ask for a medical certificate from your GP.
For more information see the DirectGov website (where this information was sourced).