Nick Clegg backed further changes to the Government's NHS reforms today as he attempted to head off Liberal Democrat anger over the shake-up.

In a joint letter with party doyenne Baroness Williams, the Deputy Prime Minister endorsed amendments designed to limit competition in the health service.

Writing to MPs and peers ahead of a crucial debate in the Upper House this afternoon, Mr Clegg insisted that changes put forward by Lib Dems would ensure the NHS "can never be treated like the gas, electricity, or water industry".

"Next month we will return to where this process all began a year ago when we meet at our party's spring conference," he wrote.

"Once these final changes have been agreed, we believe conference can be reassured that it has finished the job it started last March and the Bill should be allowed to proceed.

"We believe these changes will appeal to those in the House of Lords and the House of Commons who share our commitment to the NHS, and believe it can now embark on the reforms that matter: putting patients at the centre, working with local communities, and responding to the financial challenges of an ageing population."

Mr Clegg supported moves including scrapping proposed reviews by the Competition Commission, ensuring the NHS was "insulated" from competition law, and toughening up rules capping Trusts' income from private patients.

And he stressed that the legislation had now been amended more than a thousand times after more than 200 hours of parliamentary scrutiny.

A source close to Mr Clegg said the changes to the Health and Social Care Bill would rule out an "American-style market" in the NHS.

"We've always said there will be no privatisation of the NHS on our watch," the source said. "Nick and Shirley believe that the changes outlined in the letter will leave the Bill in the right place to be passed on to the statute book."

It was not clear if the intervention by Mr Clegg had been fully agreed with his coalition partners.

Shortly before the letter was issued, Tory health minister Simon Burns insisted the whole Government backed the Bill "as amended now".

"We don't propose to accept any more amendments, except for those that we have tabled or that are being tabled following agreement with the Government on ways to improve the Bill," he told BBC Radio 4's The World At One.

This morning, Prime Minister David Cameron's spokesman said: "We have made clear that we don't see any need for further significant changes to the Bill."

Health Secretary Andrew Lansley's flagship plans have continued to suffer criticism from professional bodies and experts despite repeated charm offensives and concessions.

The Royal College of Physicians (RCP), representing hospital doctors, is meeting today in emergency session to discuss whether to formally ballot the views of its 26,000 members on the legislation.

An online poll conducted by doctors opposed to the reforms found 92.5% of RCP members believed they should now be abandoned, according to The Observer, which was given access to the findings.

A spokeswoman for the college said: "The RCP continues to have serious concerns about the reforms and has been lobbying vigorously for changes to the Bill since its publication."

If the RCP were to come out against the legislation, it would bring it into line with the other royal colleges in calling for the Bill to be withdrawn, leaving ministers looking even more isolated.

In a further blow, former NHS chief executive Lord Crisp yesterday denounced the reform plans as "a mess", saying they were "unnecessary, confused and confusing".

Shadow health secretary Andy Burnham said: "Today's talk from the Lib Dems of minor amendments is too little, too late. They will only serve to further complicate a Bill already described as a 'confused mess' by a former NHS chief executive.

"Last week, the president of the Liberal Democrats said the Bill should have been dropped. If that's what the majority of Lib Dems believe, NHS staff will be entitled to ask why they haven't had the guts to say that to Mr Cameron's face.

"The fact is they have been sitting on the fence for far too long and have allowed this mess to develop."

Protesters calling for the Bill to be killed off chained themselves together to block Abingdon Street by the entrance to the House of Lords this afternoon.

Campaigner Shirley Murgraff, 81, from Hackney, east London was carried off by a group of police officers after refusing to move from the middle of the road.

She said: "We were there a long time, about an hour, when they came with bolt cutters and said we had to move.

"About four or five officers grabbed me under my shoulders and by my feet, I was nearly lying down, and carried me on to the pavement.

"I didn't think it was necessary but I wasn't going to move unless they made me.

"Attention needs to be drawn to the real danger that if we don't stop the Bill, that will be the end of the NHS."

Mr Cameron's spokesman said that Mr Clegg had discussed the letter with the Prime Minister over the past day.

The Prime Minister is "happy" to provide Liberal Democrats with the additional reassurance they are seeking through these amendments, said the spokesman.

"If there is a need to provide further reassurance on these issues, we are happy to provide that," said Mr Cameron's spokesman.

"This is about clarifying the position and reassuring people that we are not privatising healthcare in this country and that competition is there as a tool to improve quality for patients."

Mr Cameron's spokesman said that the PM wants his peers to continue to support the Health Bill, but indicated that this would not necessarily mean voting against the Liberal Democrat amendments.

Precise whipping arrangements for Conservative Lords are not expected to be put in place until the amendments have been tabled and their exact wording is known.

The PM's spokesman added: "He thinks it is important that we continue to push through these reforms and if that means we need to provide further reassurance, that is what we will do."

It had never been the Government's intention to use the Bill to privatise the NHS or to introduce a US-style health market, said the spokesman.

Mr Clegg told the BBC the changes he is backing "will make it clear once and for all that competition is the servant of the NHS and never its master".

He said he was in regular communication with Lib Dem MPs and peers about how to handle the controversial legislation, adding: "I explain constantly to my colleagues what I think we should do to make sure the Bill is in line with our values and our principles.

"These final amendments - having of course introduced a huge array of amendments in the past - will provide final reassurance to everybody who is anxious about this Bill that the competition provisions in it will not destabilise the NHS and will improve the quality of care for patients."

Mr Clegg said he expected to be able to secure the changes to legislation before the Bill completes its passage through Parliament.

"Then it is time to adopt the legislation and get on with the painstaking work of ensuring these changes are successfully and smoothly implemented," he said.