You do not need a doctor’s 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.
Fit notes and proof of sickness
You must give your employer a doctor’s ‘fit note’ (previously called a ‘sick note’) if you are off sick for more than 7 days in a row (including non-working days). You can get this from a hospital doctors or your GPs.
The fit note will say you are either ‘not fit for work’ or ‘may be fit for work’.
If it says you ‘may be fit for work’, your employer should discuss any changes that might help you return to work (e.g. different hours or tasks). You must be treated as ‘not fit for work’ if there’s no agreement on these changes. For more information see the DirectGov website