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 you are sick for more than seven days, your employer can ask you to provide medical evidence to support payment of SSP (statutory sick pay).
A medical certificate, now called a 'Statement of Fitness for Work’ is available from your GP if required. It is also available from other practitioners such as physiotherapists, hospital doctors and dentists.
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.
The 'fit note' was introduced on 6 April 2010. This may certify you as sick but also certify you as fit to work with your employer's support, which may mean working different hours or duties, or planning a phased return to work following an illness.
For more information see the DirectGov website (where this information was sourced)