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 of SSP (statutory sick pay).
It is up to your employer to decide whether you are incapable of work. A medical certificate ("Form MED3"), also called a 'Statement of Fitness for Work’ (see below) from any doctor treating you, is strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherwise.
It is the doctor who has been treating you who has responsibility to provide the sick note: if you need time off work after being admitted to hospital, a hospital doctor should provide it. Also for those who have been in hospital, the ward nurse can provide a sick note to certify the time in hospital. If you are an out-patient for an illness or injury preventing you from working, the doctor in clinic should provide a sick note- for example if you are under the fracture clinic for a broken ankle.
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.
Statement of Fitness for Work - ’Fit Note'
The 'fit note' was introduced on 6 April 2010. With your employer's support, the note will help you return to work sooner by providing more information about the effects of your illness or injury.
For more information see the DirectGov website (where this information was sourced)