If you started your sick leave before 10 December 2021, you must have proof if you’ve been off work for more than 7 days in a row (including non-working days).
If you started your sick leave after the 10th December you must have proof of sickness if you’re off work for more than 28 days in a row (including non-working days).
Please go to the gov.uk website where you can download a statutory sickness from which can be handed to your employer.