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

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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.

For more details on Statutory Sick Pay | nidirect