Evidence that you are sick
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.
If your illness last longer than seven days, your employer is entitled to ask you to give them a form of medical evidence to support payment of SSP (statuatory sick pay)
Employers can also ask employees to fill in a form when they return to work to confirm they’ve been off sick for up to 7 days. This is called ‘self-certification’.
If you're sick and off work for more than seven days, your employer will probably ask for proof of your illness. Most employers ask for a fit note from your GP.
However, this will also depend on your employer's company policy on sick leave (or sickness absence). This policy should tell you how many days you can be off sick before you need to provide proof of illness or a fit note.
You could also provide evidence from someone who is not a medical practitioner, 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.