Relations between the health secretary, Andrew Lansley, and Britain's doctors hit a new low on Thursday after he accused the British Medical Association of being "politically poisoned" in its opposition to his NHS shakeup.

Lansley infuriated the doctors' union by repeating a description first used by Aneurin Bevan, the founding father of the NHS, at the time the service was created in 1948. The putdown came in a pre-prepared speech in Liverpool at the launch of a new children's health initiative, rather than in an off-the-cuff remark or interview.

He criticised the growing chorus of concern from senior health professionals about his controversial health and social care bill.

However, later in the day, Lansley performed his most significant U-turn yet on the bill over the highly charged issue of the health secretary's "constitutional responsibilities" to the NHS, which a House of Lords committee had warned would be "diluted" by the proposals.

Faced by a united front – led by Labour's Lady Thornton and the Liberal Democrats' Lady Williams – health minister Earl Howe conceded defeat by withdrawing proposals that had sparked major concern and backing the alternative amendment. It explicitly states: "The secretary of state retains ministerial responsibility to parliament for the provision of the health service in England."

The health secretary has a legal duty to provide key NHS services in England, such as hospital accommodation, ambulances, maternity and nursing. Lansley had proposed that the health secretary would have only to monitor their provision and intervene in the case of services failing.

Baroness Lady Jay, the Labour chair of the Lords constitution committee, had warned that this would mean the government would no longer be legally and constitutionally responsible for the NHS. However, the concession is unlikely to prove enough to calm fears that the NHS will be dismantled by creeping privatisation.

The BMA, the Royal College of Nursing and the Royal College of Midwives have all recently changed their positions on the bill and begun calling for it to be withdrawn rather than for amendments to be made.

Speaking at Liverpool's Alder Hey children's hospital, Lansley said: "Look back to 1948 when the BMA denounced Aneurin Bevan as 'a would-be Führer' for wanting them to join a National Health Service. And Bevan himself described the BMA as 'politically poisoned people'. A survey at the time shows only 10% of doctors backed the plans."

Dr Hamish Meldrum, chairman of the BMA's ruling council, accused the secretary of state of making "inflammatory" and ill-founded remarks and ignoring warnings about the dangers of his plans.

"It is a shame that the health secretary has decided to repeat a 70-year-old myth," he said. "The BMA called for a national medical service decades before the government established the NHS in 1948. Doctors' opposition to parts of what was proposed at the time was related to the detail of the then government's initial plans for how the system would operate, not to the principle of a publicly funded and comprehensive service that was free at the point of use for all patients," said Meldrum.

"The government's current reforms threaten to fragment and disrupt patient care across our health service. The secretary of state should refrain from making inflammatory remarks and instead listen to the warnings coming not just from the BMA, but from a wide range of healthcare professionals and patients."

The war of words occurred hours before leaders of Britain's medical royal colleges and associated faculties, which represent specialist groups of doctors, gathered at the BMA's headquarters in London to try to agree a common position on the bill before it finishes its passage through parliament.

Meanwhile, it emerged that at least five layers of management will run the new GP-led commissioning system under the government's NHS reforms, prompting criticism on Thursday night that Lansley had failed to deliver his central pledge of creating a more efficient health service.

Labour said the bill, which will face a bruising ride in the Lords next month, was now "an utter shambles" after a document prepared for a meeting of the NHS commissioning board next week said there will normally be five layers of management.

The document said: "In most cases, there should be no more than five layers of management in each directorate, from national director to the 'front line'. The exception to this will be the performance and operations directorate, where an additional layer (or layers) will be required to link through to the local offices. The principle of no more than five layers of management is based on extensive evidence of effective organisational structures which has been applied in [the department of health] and other public sector organisations."

Liz Kendall, the shadow health minister, said: "Cameron's health bill is an utter shambles. He claimed it would cut costs and red tap but we now know there will be at least five layers of management with total confusion about who does what and how. None of this is in the legislation currently before parliament or been discussed by MPs. This chaos must end – Cameron should listen to doctors and nurses and drop the bill."

Downing Street is nervous that the health and social care bill, which will be considered by peers at report stage on 8 February, is in severe trouble. There is a feeling that Lansley's failure to sell his reforms to the medical profession is emboldening opponents in the Lords.