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.
Obtaining a 'Fit for Work' Note
If you are off work owing to sickness for up to seven consecutive calendar days, you need a self certificate (SC2) available from your employer or you can download a copy here. If you are self employed, you may collect an (SC1) form from the surgery. If you are off work for less than seven calendar days you do not require a doctor’s certificate. Any absence for more than seven days, including weekends and bank holidays, requires a 'Fit for Work' certificate, available from your doctor free of charge.
If your employers require a certificate for a period of less than the statutory seven days, the doctor is not obliged to issue any certificate but may issue a private certificate for which there is a fee (current charges are available from reception).