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. Visit the following link for information and the form for self-certification.
If your employer insists on a doctor's certificate during the first week of illness, this will be issued in the form of a private fot / sick note, for which there is a charge.
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 / sick note (or Statement of Fitness for Work) from a GP or hospital doctor. Fit / sick notes are sometimes referred to as medical statements or a doctor's note.
For more information please visit;
When do I need a fit / sick note? (NHS information)
The fit note: guidance for patients and employees (GOV.uk)