More than 60 leading medical professionals have demanded that the Government's "unpopular and undemocratic" health reforms, which return to Westminster today, be either scrapped or heavily rewritten.

Their call, in a letter to The Independent, increases the pressure on Andrew Lansley, the Health Secretary, who faces widespread opposition in the House of Lords to the planned overhaul.

Signatories include consultants, surgeons, psychiatrists and paediatricians, as well as midwives and family doctors. They have joined forces with public figures including the actors Julie Christie and David Morrissey, the comedian Russell Brand and the fashion designer Dame Vivienne Westwood to demand a halt to the reforms. They say that, despite a series of amendments to the Health and Social Care Bill, Mr Lansley's plans still do not have the support of the public or the profession.

"It is perfectly clear – as the Prime Minister is acutely aware – that the British public does not support the privatisation of the NHS, and it is a matter of fact that no one ever voted for it, and so this current Bill has no democratic mandate whatsoever," they write.

They call for the "suspension of, or significant amendment of, the Bill in order that it can be supported by a majority of the medical profession and the British public as a whole, who pay for, support and service our great NHS". The signatories add: "No one is against reform and change, but the NHS is too important and valuable to our society to be transformed forever in this unpopular, undemocratic way."

Their letter comes as the Bill returns to the House of Lords where it is due to face concerted opposition, with so many peers requesting to speak that business managers have been forced to set aside another day for the debate.

More than 90 members have applied to speak in the Second Reading debate. Attempts will be made to block the Bill or to delay it by referring key parts to a select committee.

Ministers are expected to offer one concession – an amendment designed to spell out more clearly that the Health Secretary has ultimate legal responsibility for the NHS. But there is growing anxiety within the Government that critics could muster enough support to delay the measure.

David Cameron was forced to announce a halt to the Bill's progress in April in the face of widespread hostility among health professionals, as well as Liberal Democrat activists. Following a six-week "listening exercise", he announced changes to the legislation, including involving clinicians in handling budgets and a retreat from plans to promote competition.

But the moves have failed to appease many of the Bill's critics, with the Royal College of Nursing and the British Medical Association still strongly opposed to it. The Academy of Medical Royal Colleges wrote to peers yesterday arguing that too much emphasis is being placed on competition and choice at the expense of patient care.

One of the signatories of the letter to The Independent, Peter Fisher, the president of the NHS Consultants' Association, said: "Our main concern is that it opens the health service to much greater private involvement." He added: "It could become a collection of individual organisations – some NHS, some private, some voluntary – competing with each other. The essence of medicine is co-operation, not competition."

Key signatories behind the letter

Nigel Speight

The consultant paediatrician from Durham has forecast the "destruction of the NHS" if the reforms are pushed through, claiming that doctors and hospitals could soon be forced to compete in a "patients' marketplace".

Dr Alex Scott-Samuel

A lecturer in public health at the University of Liverpool, he has warned the Bill would do "irreparable harm to the NHS, to individual patients and to society as a whole" and points out that it is highly unusual for NHS staff to speak out publicly in this way.

John Puntis

The consultant paediatrician from Leeds claims the Bill is "propelling us towards a US-style market economy in health, with all the inefficiency, waste and poorer outcomes that are well-known".

Russell Brand

The comedian has criticised US healthcare, remarking: "Instead of letting people die in the street we have free healthcare."

House of Lords: obstacles to reform

Lord Rea

The Labour peer Lord Rea, a former junior hospital doctor, GP and lecturer at St Thomas' Hospital Medical School, has tabled a motion to stop the Bill going any further in the Lords.

Lord Owen

The crossbencher Lord Owen, a former hospital doctor and a health minister from 1974 to 1976, is leading moves for key clauses to be considered by a select committee, thereby halting the Bill's overall progress.

Baroness Williams

The former Liberal Democrat leader in the Lords, Baroness Williams of Crosby, has called for further concessions, raising particular concerns that Bill does not enshrine in law the Health Secretary's responsibility for the NHS.

Lord Hutton