Ministers face another wave of opposition to the coalition's NHS reforms, which return to the floor of the Commons next week.
The British Medical Association's (BMA) chairman of council Hamish Meldrum has written to MPs insisting that the watered-down proposals still present an "unacceptably high risk to the NHS".
Meanwhile Labour have accused the government of "railroading" the health and social care bill through parliament without giving it proper scrutiny, ahead of its return to the Commons floor next week.
This spring David Cameron, Nick Clegg and health secretary Andrew Lansley were forced into a humiliating climbdown on large parts of the controversial reforms.
They abandoned plans to make the regulator Monitor focus solely on competition and hand commissioning powers to GPs exclusively after a lengthy 'listening pause'.
Despite the reversals the BMA, which led the campaign against the shift towards a market dynamic, remains unconvinced.
"The BMA acknowledges the efforts of government to listen to and address some of the concerns that have been expressed about the bill to date," Dr Meldrum wrote in the letter released today.
"However, we still believe that the government's reform plans pose an unacceptably high risk to the NHS, threatening its ability to operate effectively and equitably, now and in the future.
"This is why the BMA continues to call for the bill to be withdrawn or, at the very least, to be subject to further, significant amendment."
It objects to the removal of the cap on how much foundation trusts can generate from private patients, fearing that access to NHS patients could be cut as a result.
Other areas of concern include forcing all NHS trusts to become foundation trusts, uncertainty over whether the secretary of state will have ultimate responsibility for a comprehensive health service and the proposed 'quality premium' for commissioners - which may be judged against financial performance, not well-defined quality measures.
"We believe there continues to be an inappropriate and misguided reliance on 'market forces' to shape services," Dr Meldrum added.
"It is also implicit in the bill, which embeds a more central role for choice without a full consideration of the consequences and which creates ambiguity about how the trade offs between increasing patient choice and ensuring fair access, integrated care and improved efficiency should be managed."
The health and social care bill receives its report stage and third reading in the Commons next Tuesday and Wednesday, before moving to the Lords.
Further resistance to the reforms in parliament would be of serious concern to senior coalition figures, who had hoped the unprecedented reversals earlier this year had sufficiently addressed health workers' concerns.
Shadow health secretary John Healey said Mr Cameron was trying to rush the legislation past MPs without the bill being given proper scrutiny.
"Limited time for debate and the lack of a new impact assessment, following the Future Forum report, means that MPs are being asked to judge the bill in rapid time and without the full facts at their disposal," he said.
"By rushing the parliamentary process, David Cameron's government is forcing through bad legislation that will lead to more waste, cost and confusion in the NHS.
"MPs should be given more time to scrutinise and debate the bill, alongside an updated impact assessment."