The warning came despite a GPC pledge that commissioning and provision of out-of-hours services would be kept separate.
A motion calling for GPs to have a central role in commissioning of out-of-hours services divided representatives at the 2010 conference of LMCs.
It was narrowly carried, with 52 per cent of delegates in favour.
But North Yorkshire LMC member Dr Phil Garnett warned that taking on commissioning of out-of-hours was a 'poisoned chalice' that would inevitably lead to GPs providing the service as a last resort.
He said: 'Once you take responsibility for commissioning and get involved, if you fail in the tendering process - if you fail to get a provider or find a good enough one - it's inevitable you feel a responsibility to provide the service yourself.'
As a caring profession, many GPs would feel unable to refuse to provide the service if they could not commission a good alternative, he said.
But GPC member Dr Nigel Watson, chief executive of Wessex LMCs, said the government has 'reaffirmed several times' that commissioning is not the same as being a provider of last resort.
'Any negotiations (with government) will have to see those things separately,' he said.
'Out-of-hours is a key issue for many of us, and this is one of the areas where there is a line in the sand.'
LMC representatives also carried parts of a motion that called for the GPC to 'resist vigorously' any attempt to make GPs providers of last resort, and for the government to invest in out-of-hours services.
But a separate part of the motion calling for the GPC to publish a set of criteria, independent of the RCGP, that would apply if GPs take on responsibility for out-of-hours commissioning, was lost.
Dr Watson said: 'I don't see why we should exclude the RCGP from any discussions. I think that would be counter-productive.'