The health secretary will perform a dramatic climbdown over his reforms this week in a desperate attempt to prevent a cross-party revolt among peers who fear that the changes would lead to the fragmentation of the NHS.

Amid growing concern in Downing Street that health policy is becoming the government's achilles heel, ministers will table a series of amendments to the health and social care bill that will oblige Andrew Lansley to maintain the NHS as a national public service and, his critics say, limit his ambitions to expand the role of the private sector.

The changes will also spell out the kind of services that must be offered by GPs and will effectively ban them from withholding certain forms of care from patients.

On Saturday Labour's health spokesman in the House of Lords, Baroness Thornton, described the move as a "massive climbdown" by Lansley. But she said the bill still remained deeply flawed and that attention would turn to clauses dealing with plans to increase competition when it returns to the Lords next month.

The peers, led by the Lib Dem, Baroness Williams, and supported by a former Tory lord chancellor, Lord Mackay of Clashfern, have complained that the original bill left serious legal doubt as to whether the secretary of state would any longer be responsible for providing a "comprehensive health service for the people of England free at the point of need".

They feared that the absence of a chain of accountability would allow the service to become fragmented as different groups of doctors adopted different approaches and the role of the private sector expanded.

Lansley's reforms will abolish two major tiers of health service bureaucracy and devolve greater responsibility for commissioning care to GPs – moves the health secretary believes will deliver a more efficient service and a system of care tailored better to patients' needs.

The Department of Health confirmed the changes would be made to the bill but denied they were a panic response following a fortnight in which Lansley's approach has been criticised by a cross-party group of MPs and a growing number of health professionals.

One of the amendments was sparked by concern that the new consortia of local doctors in each part of England would be able to deny patients certain treatments because of their lifestyles. In Hertfordshire, the Herts Valley Clinical Commissioning Group (CCG) has become the first in England to tell obese patients to lose weight or they would not receive gall bladder, hernia or tonsil surgery. The CCG has also told smokers that they have to see a counsellor about trying to quit before they can undergo certain operations.

Although the NHS already imposes conditions on certain patients, there was concern that CCGs would go much further if the health secretary was no long responsible for ensuring they provided a national service.

A letter from the government health minister Lord Howe to a group of peers last week confirmed the changes. It said that "there seems to be an emerging consensus about how the bill can be improved in order to put beyond doubt the secretary of state's accountability for the health service".

Meanwhile, opposition from doctors to the bill appears to be growing. The Royal College of Physicians, which represents hospital doctors, is under pressure from members to hold an emergency general meeting. The members want it to follow the British Medical Association, the Royal College of Nursing and the Royal College of Midwives in calling for the bill to be scrapped.

The body representing NHS radiologists has also voiced "grave concerns" about the bill and said that: "Given our widespread concerns over many serious and as yet unresolved issues, the Royal College of Radiologists cannot support, and must continue to oppose, the passage of the bill in its current form."

The Royal College of Psychiatrists has taken a similarly hard line while the College of Emergency Medicine, which speaks for A&E doctors, has also voiced serious doubts.

Andy Burnham, the shadow health secretary, said on Saturday that there had been "sheer panic" at the Department of Health. "But no amount of pressurising phone calls and desperate concessions will make the bill acceptable," he said.