Responding to the Department for Work and Pensions' (DWP) consultation on the new forms, the GPC said tick boxes on the forms could be easily tampered with.
The revised MED3 forms will expand the role of the traditional sick note to include suggestions about what tasks a patient is capable of doing at work.
But GPC chairman Dr Richard Vautrey warned that the proposed form contained some basic flaws and the issue of workplace health was complex.
'We recognise the benefits of work as a way of promoting health,' said Dr Vautrey.
'We are not averse to a change in the arrangements, but bear in mind, GPs are not occupational health physicians.
'It's impossible for a GP to know what a patient's workplace really is like.
'It's not suitable for a GP to give advice to an employer that could potentially place it in breach of health and safety requirements.
'One of the proposals was to have simple tick boxes - if that was the case, someone could simply add a tick to one of the boxes after the consultation. We'd want to ensure any form was not open to fraudulent use.'
The DWP hopes to roll out fit notes by 2010.
- To view the note and take part in the consultation, visit http://research.dwp.gov.uk and search for 'fit note'