If you have an illness lasting seven days or less you do not require a doctor's sickness certificate. Your employer may require you to complete a self-certification form called an SC2 which is available from the HMRC website or you can click on the link below to download a form.
Self certification sickness form SC2
If you are off work for more than seven days due to illness, 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.
Statement of Fitness for Work - 'Fit Note'
The 'fit note' was introduced on 6 April 2010. With your employer's support, the note may 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 (from where this information was sourced)