Conservatives are digging their own electoral graves by pressing ahead with NHS reforms in the face of growing opposition from health professionals, Labour leader Ed Miliband said today.

He was speaking after the first GP commissioning group called on David Cameron to scrap his Health Bill, warning that it was getting in the way of their work and was not needed to improve services to patients.

During rowdy exchanges in the House of Commons, the Prime Minister played down opposition from professional bodies to his reforms, pointing out that in some cases only a small proportion of members had taken part in votes against the Bill.

As the Health and Social Care Bill returned to the House of Lords for the latest day of a protracted and fractious debate, Health Secretary Andrew Lansley received a setback as the Tower Hamlets Care Commissioning Group (CCG) went public with its opposition to the legislation.

The east London group has been at the forefront of the move to GP-led commissioning, which is at the heart of the Bill, and its chair Dr Sam Everington hosted Mr Lansley for his first speech as Health Secretary.

But the CCG wrote to Mr Cameron today to tell him he was wrong to claim that GPs' participation in the reforms meant they backed the Bill, which they said should be ditched.

"Your rolling restructuring of the NHS compromises our ability to focus on what really counts - improving quality of services for patients, and ensuring value for money during a period of financial restraint," wrote the GPs.

"We care deeply about the patients that we see every day and we believe the improvements we all want to see in the NHS can be achieved without the bureaucracy generated by the Bill.

"Your Government has interpreted our commitment to our patients as support for the Bill. It is not."

Mr Miliband seized on discontent over the health reforms for the fourth week in succession as he challenged Mr Cameron at Prime Minister's Questions in the Commons.

Quoting Dr Everington's concerns, the Labour leader told the Prime Minister he had "lost the confidence even of the GPs he says he wants to be at the heart of these reforms".

The Bill has been described by former NHS chief executive Lord Crisp as "a mess... confused and confusing" and there have been calls for its withdrawal from the Royal Colleges of GPs, nurses, midwives and radiologists as well as the Faculty of Public Health, Chartered Society of Physiotherapy, Community Practitioners and Health Visitors Association, and Patients Association, said Mr Miliband.

"Doesn't it ever occur to him that, just maybe, they are right and he is wrong?" he asked.

And he directed a barb at Conservatives who jeered him across the chamber: "Their support for the Health Bill is digging their own burial at the next general election."

But Mr Cameron accused the Labour leader of taking a "totally opportunist position" on the Bill, when he accepted that reform was needed.

Just 7% of the Royal College of GPs' 44,000 members and 2% of 50,000 physiotherapists had voiced opposition to the Bill when their professional organisations surveyed members' views, said the Prime Minister.

"I know that's enough for the unions to elect a leader of the Labour Party, but that's about as far as it will go," he said.

Mr Cameron listed the National Association of Primary Care, the NHS Alliance, the Association of Chief Executives of Voluntary Organisations and the Foundation Trust Network as bodies which back his Bill.

And he accused Mr Miliband of dodging debate on the substance of reforms: "Four weeks in a row of NHS questions, but not a single question of substance - not one.

"All about process, all about politics, never about the substance... We all know what he is against, but isn't it time he told us what on Earth he is for?"