You do not require a doctor's sickness certificate for any illness lasting seven days or less. However, if you have been sick for more that four days in a row, but less than seven, your employer may ask you to self-certify your illness using a SC2 form. This form 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). This will be a Fit Note or a Statement of Fitness for Work from your GP or Hospital Doctor. Please be advised that if you are asked by your employer to provide a certificate other than as set out you may be asked by the Practice to pay for a private certificate.
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)