The government proposes to introduce the initiative in spring 2010 despite a Londonwide LMCs survey of 452 GPs finding that 96% have not been trained to carry out the work.
Asked: 'Do you feel confident that you have the expertise to accurately complete fit notes for your patients?', 79.9% answered no, 9.3% yes and 10.8% did not know.
95% of GPs do not feel that they are the appropriate person to complete Fit Notes and 81% think that occupational health doctors are best able to make such complex judgements about a patient's ability to work in certain roles.
Dr Michelle Drage, joint chief executive of Londonwide LMCs, said: 'We are our patients' advocates and taking on this role would jeopardise that relationship.
'We are here to make assessments about our patients' illnesses, not divert our scarce resources into running an occupational health service for employers at our patients' and the taxpayers' expense. We hope the government will rethink this proposal in the interest of patient care.
'There are elements of this that may benefit all parties - patients, GPs and employers, but it needs really serious reworking with the profession.'
