Sick for less than 7 days
You do not require a doctor's sickness certificate.
Your employer may require you to complete a self-certification form (SC2) which is available from your employer or on the HMRC website.
Sick for more than 7 days
Your employer may ask for medical evidence to support payment of Statutory Sick Pay.
It is up to your employer to decide whether you are incapable of work. A medical certificate, now called a 'Statement of Fitness for Work’ from your doctor is strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherwise.
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.