Government plans for radical reform of the NHS in England were today branded "potentially disastrous" by leaders of doctors and nurses.
In a letter to The Times ahead of Wednesday's publication of the Health and Social Care Bill, the heads of six health unions, including the British Medical Association and the Royal College of Nursing, warned of their "extreme concerns" about plans to create greater commercial competition between the NHS and private companies within the health service.
The speed and scale of the reforms proposed by Health Secretary Andrew Lansley risks undermining the care of patients by putting cost before quality, they said.
The letter follows a report by the NHS Confederation which acknowledged the potential benefits of the changes, which will give GPs power over commissioning treatment worth 80 billion, but warned that they were "extraordinarily risky" at a time when the NHS is losing 45% of its management resources and facing its toughest financial constraints for a decade.
In a speech in London today, Prime Minister David Cameron will give his strong support to the reforms, speaking of his political and personal "passion" for modernisation and declaring that "we should not put this off any longer".
He will announce that 141 GP consortia, serving more than half of the population of England, have now signed up as "pathfinders" to pilot the new arrangements ahead of their planned implementation in 2013.
But shadow health secretary John Healey said that the NHS Confederation report was a "big red warning light" of the dangers of the reorganisation plans, which he described as "a cocktail of instability and uncertainty laced with broken promises".
And the Times letter, whose signatories include BMA chairman Hamish Meldrum, RCN chief executive Peter Carter and the heads of health for the Unison and Unite unions, questioned the decision to carry out wholesale changes at a time when the NHS is being asked to save 20 billion.
The letter warned: "There is clear evidence that price competition in healthcare is damaging."
Today's NHS Confederation paper - reporting the conclusions of a summit of healthcare managers, doctors' leaders, patients groups and policy experts last November to discuss the implications of Mr Lansley's proposals - described the reforms as "undoubtedly the biggest shake-up of the NHS in its history".
Unless large numbers of GPs get actively involved in the new system and engage in the strategic challenges of changing their local health services, there is a danger of "potentially overwhelming" pressures leading to "widespread financial problems... sub-optimal performance and longer waiting times", it warned.
The reforms could produce "some very beneficial effects for patients and population health", but ministers have "systematically under-estimated the very significant cultural and behavioural changes required, in particular from GPs".
"For any chance of success, there needs to be a high level of GP engagement and consortia will need to be ready to take up their role very quickly," said the report.
And it warned: "While participants were able to imagine the reforms being successful, they said it requires that a degree of optimism and a number of demanding conditions will need to be met.
"Firstly, there are issues about the reforms themselves that will need to be addressed. Secondly, and of much more concern, is the timetable for the dismantling of one system, its replacement with another and the delivery of a number of other very challenging tasks, which seems to carry a very high level of risk."
Mr Healey described the report as "a comprehensive demolition job on the Conservative-led Government's handling of the NHS".
"Ministers are putting unprecedented pressure on the NHS by forcing through this high-cost, high-risk internal reorganisation and breaking their promise to protect NHS funding," he said.
But Mr Lansley said that the rapid take-up of pathfinder status showed that GPs were keen to take on new responsibilities.
"It is clear that GPs and nurses are ready and willing to take on commissioning responsibilities," he said. "The pathfinders to date demonstrate this but, most importantly, the changes will enable them to make the decisions that better meet the needs of their local communities and improve outcomes for their patients."
He cited a pathfinder consortium in Croydon which set up mobile screening clinics for heart monitoring and ultrasound, speeding up diagnosis and treatment and saving patients' time, and another in Somerset which reduced hospital admissions by 46% by improving prescribing and giving nursing home residents more choice on end-of-life care.
NHS Confederation acting chief executive Nigel Edwards said: "We see real potential benefits in both moving decision-making closer to GPs and in extending the range of providers in order to drive efficiency and innovation.
"Success is possible but the Government needs to sell its vision and show that it has the ability to drive change. That will mean developing an acute understanding of the risks and how to manage them."