The UK's largest medical royal college has called for the prime minister to scrap the health and social care bill, branding it "damaging, unnecessary and expensive".
The Royal College of General Practitioners (RCGP) has written to David Cameron following the tabling of amendments to the controversial bill in the House of Lords this week.
They said that despite the amendments, they believed the planned reform would "cause irreparable damage to patient care and jeopardise the NHS".
The RCGP chairwoman, Dr Clare Gerada, said: "This decision was not taken lightly, but it is clear that the college has been left with no alternative.
"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 three-quarters of respondents to a recent poll said they thought it appropriate to seek the withdrawal of the bill.