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 request a note within the first seven days then this will be issued as a Private medical certificate and will be charged at £25.00.
Evidence that you are sick
If you are sick for more than seven days, your employer will ask you to give them some form of medical evidence to support payment of SSP (statutory sick pay).
The 'fit note' was introduced on 6 April 2010 which replaced the old 'Med3' and 'Med5' certificates.
The doctor will need to have seen you during your illness to be able to issue this note and it cannot be backdated unless the doctor has written evidence from another clinician who has seen you during this time.
The doctor will decide an appropriate amount of time for you to be off work and will let you know whether you need to be reviewed again prior to your return to work.
For more information see the DirectGov website