It is up to your employer to decide whether you are incapable of work.
A medical certificate, now called a 'Statement of Fitness for Work’ is strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherwise.
You could also provide evidence from someone who is not a GP, e.g. a Dentist.
For a sick line to be issued by us, you must have spoken or been seen by a GP
For an extension on your sick line you may not need to be seen again. Please ask the receptionist for more details.
Statement of Fitness for work - 'Fit Note'
The 'fit note' was introduced on 6 April 2010. With your employer's support, the note will help you return to work sooner by providing more information about the effects of your illness or injury.
For more information see the Direct Gov website.