An overwhelming majority of LMC representatives voted in favour of a motion claiming core GPs' work will be undermined by the Health Act at this year's LMCs Conference in Liverpool.
LMCs also approved motions stating that the Act will widen health inequalities and compromise the health of the nation.
BMA leadership managed to avoid a vote on its handling of the association's opposition to the Health Bill, following criticism that its response was too slow.
Proposing the motion that core general practice was under threat, Dr Gerard Reissmann of Newcastle and North Tyneside LMC said patients would lose trust in their doctor if they see them as the rationers of care.
'At the heart of general practice, it is about our relationship with patients. As the Bill is enacted and becomes what we do in our everyday lives, patients will never trust us again. That's the US experience in the 1980s as doctors embraced the market.
'With that loss of trust goes core general practice. Patients will see it, that we are cutting their services.'
But a motion proposed that all GP practices should consider withdrawing from involvement in clinical commissioning groups (CCGs) and other NHS management was defeated by representatives.
Dr Reissmann had argued: 'How can we let our patients know that we don’t embrace the market, as appear many of us don't? The answer is that beyond the legal minimum requirement, that you and your practice should not be involved.'
GPC negotiator Dr Chaand Nagpaul agreed that the Health Act threatened core general practice. He said: 'The fabric of this Bill is rooted in competition, fragmentation and marketisation. The Bill... will impact on the lives of GPs.'
But he rejected the call for all practices to consider withdrawing from CCGs.
Dr Nagpaul said: 'We do need to support GPs to consider withdrawing from commissioning if the price is too great. That needs to be a clear message to government: they cannot take our support for granted if it threatens our professionalism, undermines the trust of our patients, and destroys the NHS in the process.'
'But many doctors do think it legitimate to be involved in commissioning for their patients. GPs' involvement in commissioning should challenge government policy and make sure we don't get privatised commissioning in the health service.'