Employees must give their employers a doctors fit note (sometimes called a sicknote) if they've been ill for more than 7 days in a row and have taken sick leave. This includes non working days such as weekends.
If employees are self isolating due to Covid-19, they can get an isolation note from NHS 111.
Fit notes are free. Your GP may charge a fee for any notes required earlier than 7 days. Id employees are off sick for less than 7 days, they do not need to give their employer a fit note. When they return to work, their employer may require them to confirm they have been off sick. This is called a self certification. Your GP cannot provide self certification.