The UK's largest medical royal college of GPs has launched a scathing attack on proposed NHS reforms, branding them "damaging, unnecessary and expensive".

The Royal College of General Practitioners (RCGP) has written to David Cameron over the Prime Minister's controversial Health and Social Care Bill, after the tabling of amendments to the bill in the House of Lords this week.

Focussing on the risks they believed it posed to patients, they said that despite the amendments, the planned reform would "cause irreparable damage to patient care and jeopardise the NHS".

RCGP chairwoman Dr Clare Gerada said: "We have taken every opportunity to negotiate changes for the good of our patients and for the continued stability of the NHS, yet while the Government has claimed that it has made widespread concessions, our view is that the amendments have created greater confusion.

"We remain unconvinced that the bill will improve the care and services we provide to our patients."

The college, which represents more than 44,000 family doctors, said that three-quarters of respondents to a recent poll said they thought it appropriate to seek the withdrawal of the bill. They wrote to Health Secretary Andrew Lansley to voice the concerns of their members but decided to take action after receiving his response, and following the Government's tabling of amendments on Wednesday.

Dr Gerada said: "Competition, and the opening up our of health service to any qualified providers will lead not only to fragmentation of care, but also potentially to a 'two tier' system with access to care defined by a patient's ability to pay."

The 20 colleges that make up the Academy of Medical Royal Colleges have been divided over the strength of the stance they should take against the Bill. Those opposing it include the Royal College of Radiologists, which said it had "grave concerns", and the Royal College of Psychiatrists, which called the bill "fundamentally flawed".

The Government has been criticised for failing to allay fears over an increased role for private companies in running the NHS. A critical report from MPs on the Health Committee last month said the overhaul was hindering the ability of the NHS to make the savings it needs to safeguard its future.

One of the amendments laid out this week said the new NHS Commissioning Board and clinical commissioning groups run by GPs would have new responsibilities to support education and training. Both will also have to report annually on their progress in tackling health inequalities, together with the Health Secretary.

Mr Lansley said the Government had been "carefully listening" to opinions about the Bill and that the series of amendments would "address these remaining issues".