Illness lasting <7 days

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 your employer insists on a certificate for an absence of less than seven days, this can only be provided as a private certificate for which a charge will be made.

Illness lasting >7 days

For any illness lasting longer than seven days you will usually need to speak to the GP in order that they can issue a Fit Note (MED3) and for any subsequent continuation certificates.

'Signing off' certificates are not required.