It is a requirement for an ESA claim to progress that the claimant is able to provide a statement that they are Unfit for Work, the simplest version of this is a Fit Note. If you don't provide one then the ESA system will not release payments and it is possible that you claim will be refused.
Speak to your GP about getting a Fit Note.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems
