If you are ill for more than seven days, your employer can ask you to give them some form of medical evidence.
You do not need a doctors Sickness Certificate or Fit Note 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 wesite.
Patients can get a Fit Note from a GP or hospital doctor. If your employer agrees, you can also provide evidence from someone who is not a medical practitioner such as a dentist, physiotherapist, podiatrist or occupational therapist instead.
For more information please see the DirectGov website.