The controversial Bill, which is currently in the House of Lords, has already been heavily amended. But the Liberal Democrat leader is now focusing on plans to increase the number of providers in the health service and to give a review role to the Competition Commission, which regulates markets in the private sector.

In a letter sent to Liberal Democrat MPs and peers, Clegg said the proposed amendments, which are likely to be debated later this week, would ‘make sure that the NHS is never treated like a private industry’ in terms of competition.

This would end ‘any threat of takeover from US-style healthcare providers by insulating the NHS from the full force of competition law’, he added.

The letter, which was also signed by senior LibDem Baroness Shirley Williams, called on peers to back the amendments, which include scrapping the role of the Competition Commission and making foundation trusts seek permission from governors before carrying out extra private work.

Clegg said the Bill had already changed from the one that LibDem members had voted against at last year’s spring conference. Following the government’s consultation on the plans last summer, the NHS Future Forum had been set up and had recommended a number of alterations to the legislation.

Clegg said yesterday that ‘the Bill should be allowed to proceed’ if these final changes were agreed.

Commenting on the letter, a Department of Health spokesman said: ‘This Bill has never been about introducing an American-style insurance system. But of course we will continue to work with peers to provide the assurances sought.’

The publication of Clegg’s letter came as it was confirmed hospital doctors are to be given a vote on whether they back the government’s plans. At an extraordinary general meeting yesterday, the Royal College of Physicians backed motions calling for the vote.

The college is the latest health care organisation to consult its members on the government’s reforms. It follows the Royal College of Paediatrics and Child Health which last week called for the government to withdraw the changes. The Royal College of GPs and the British Medical Association have also called for withdrawal.

At yesterday’s meeting, the 189 attending fellows of the Royal College of Physicians voted by 80% to 16% to survey members on their views on the Bill.

Another non-binding vote, which urged the college to call publicly for complete withdrawal of the Bill, was agreed by 79%. This will be sent to the organisation’s council for consideration, RCP president Sir Richard Thompson said.

The DoH spokesman said that the Bill had already been ‘strengthened’ by the Future Forum’s recommendations. These had responded directly to the RCP’s concerns, including clarity that competition would be used only to benefit patients.

He added: ‘We will continue to work with the royal colleges throughout the passage of the Health Bill through Parliament.’