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. Your employer is not entitled to ask for a Fit Note for absences of 7 or less calendar days, and if they insist there will be a charge for a Private Medical Certificate.
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). This is called a Fit Note ( previously a Medical Certificate or Sick Line). This is completed by a GP and usually needs a consultation which occasionally can be by telephone. Please ask your employer what they require or consult you Sickness Absence Policy.
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 Fit Note 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 may help you return to work sooner by providing more information about the effects of your illness or injury. You do not need a certificate to say you are fit to return to work. The certificate lapses at the end of certified period. You may return to work before the certified period if you are well and the certificate will automatically lapse. Where there is a dispute about somebody's fitness to work then the employer needs to consult an Occupational Health Service. The Employer is not entitled to ask for a report from the patients GP, but with the patients consent can ask for a report and there will be a charge for a Private Report.
For more information see the DirectGov website (where this information was sourced)