GP leaders warned that the RCGP's intention to add a fourth year of training as it stands may force trainers to foot the bill and lead to poorer quality teaching for trainees' final year.
They said they could not back the scheme unless medical deaneries guarantee funding and avoid 'an educationally reasonable idea' being spoilt by a lack of resources.
GPC chairman Dr Laurence Buckman said: 'Our problem with [the plan] is that it doesn't appear to have a sound financial footing.
'We are concerned trainers may have to foot the bill, and trainees may not necessarily get good teaching during the fourth year because there won't be any money to pay the teachers to teach them.'
He added: 'What we want is not just a year tacked on, but a coherent plan from the moment you enter to moment you leave GP training, that this extra year is not just time served but there is a point in doing it.'
Dr Buckman concluded: 'We can't support a scheme that isn't funded.'
GPC deputy chairman Dr Richard Vautrey said: 'What we're concerned about is the potential knock-on effect, at least to trainers grants or more generally, and also the particular posts that fourth year trainees might find themselves in.'
He said posts adopted by trainees on their fourth year will in many cases provide good education and experience. But the fear is they may end up effectively backfilling posts, to the exclusion of salaried GPs.
Dr Vautrey said the government needs to 'invest in the workforce to ensure it gets best qualified GPs in the future'. ‘There's no reason why we cannot plan now for investing in the future of general practice under the NHS by 2016,' he said.
The GPC warned in January that proposed guidance from the Committee of General Practice Education Directors (COGPED) and the RCGP for extended GP training failed to clarify the educational benefit to trainees.
A spokeswoman for the RCGP said: 'We have set up a working group specifically to look at implementation and how the extra year can be funded. We are grateful to have the support of the GPC, COGPED and the Conference of Postgraduate Medical Deans (COPMeD) on this group and a number of options will be considered before we submit our bid later this year.'