When do I need a sick / fit note?
7 days off sick or less
If you're off work sick for 7 days or less, your employer should not ask for medical evidence that you've been ill. Instead they can ask you to confirm that you've been ill. You can do this by filling in a form yourself when you return to work. This is called self-certification. Information and the form for self-certification.
More than 7 days off sick
If you're off work sick for more than 7 days, your employer will usually ask for a fit note (or Statement of Fitness for Work) from a GP or hospital doctor. Fit notes are sometimes referred to as medical statements or a doctor's note.
For more information please visit; https://www.nhs.uk/nhs-services/gps/getting-a-fit-note/