The Statutory Sick Pay Regulations state that employers are required to accept self-certification notes (filled in by you) for the first seven days of illness.
These should be available from your employer or you can download the Statutory Sick Pay (SSP) Employee’s statement of sickness from the HM Revenue & Customs website.
Anything over seven days means you will need a doctor’s certificate and will need to visit a Doctor to acquire this. This certificate is known as a Statement of Fitness for Work (or Fit Note). 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 visit the DirectGov website.