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, now called a 'Statement of Fitness for Work’ (see below) 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.
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).
Examinations and sickness certificates
It should be noted that GPs do not provide sick notes for schoolchildren. When children are absent from school owing to illness, schools may request a letter from a parent or guardian, and this is no different during an exam period. However, children who have missed exams due to illness are frequently told by schools that a note from a doctor is required; but this cannot be provided by a GP. Aside from the fact that parents/guardians are responsible for excusing their children from school, GPs cannot provide retrospective sickness certification. When a child suffers from a long-term condition, any certification will be provided by the responsible specialist.
The Office of the Qualifications and Examinations Regulator states that Awarding Organisations make no requirement for pupils to obtain a medical certificate in support of their application for special consideration. Students are asked for information in support of their application, but this may take the form of a statement by the school. The Joint Council for Qualifications has confirmed that as far as they are concerned, if a student was absent from an examination as a result of illness and has the support of the school or centre to be absent, special consideration will be granted on that basis. Awarding organisations do not insist that medical proof is provided