Hospitals will have to close, patient care could be hit and treatment rationed by GPs because of the government's controversial shake-up of the NHS, health bosses and medical leaders have warned.

The biggest restructuring of the service since its creation in 1948 is described as "extraordinarily risky" by NHS leaders and medical groups in a new report.

The analysis by the NHS Confederation – comprising the British Medical Association, the Faculty of Public Health and the royal colleges representing GPs, surgeons and hospital doctors – comes ahead of publication of the government's flagship Health and Social Care Bill on Wednesday.

The report accepts the need for reform but criticises the health secretary, Andrew Lansley, for failing to persuade patients and health professionals that his radical proposals to hand the power to commission services to GPs will improve the NHS, and for not doing enough to boost patient power. Their intervention is another blow to Lansley, whose overhaul of the NHS in England has been criticised as unnecessary, reckless and too far-reaching.

On Monday, amid anxiety about the plans in the Tory-Liberal Democrat coalition, David Cameron will deliver a speech in which he will try to soothe worries of MPs and medical professionals. The prime minister is expected to say that the plans to devolve power to GPs will not be rushed and that lessons will be learned from pilot projects.

On Tuesday, Lansley will face further problems with publication of a searching report by the all-party health select committee. Sources say the committee will be "robust" in its analysis and ask a string of "searching questions". The committee will recommend how the new system could deliver better value for money, given the budget restraints now facing the NHS.

Before the election the Tories promised there would be no major "top-down" reorganisation of the NHS, only to unleash proposals for the most sweeping changes for decades. The conclusions of the confederation make uncomfortable reading for Lansley.

"The absence of any compelling story about why the reforms are necessary or how they will translate into improved outcomes is of concern," states the document. It also criticises ministers' "unpleasant and demotivating" attacks on NHS managers, whom they are purging while expecting them to drive through the reforms.

There is also disquiet in Lib Dem ranks. One senior Lib Dem said the Lansley project was a leap in the dark: "There are three scenarios: it could be a disaster; it could be just about OK; or it could work. I can't think of a major reform where so many have regarded the outcome as so uncertain."

The prime minister is now said to be determined to take a close interest in the passage of the NHS bill through parliament and to examine the concerns of the medical profession.

The report raises a series of worries about the potentially negative impact of the reforms. It says the switch to a system based on "any willing provider", in which new consortia of GPs will be able to send patients to whoever offers them the best treatment, will force the NHS to shrink in order to make space for new private healthcare providers.

While introducing such market mechanisms can improve quality and efficiency of care, it says, "this will not happen naturally when, as in the case of the NHS, the size of the total market is not increasing. Closure of existing services will be necessary."

The report urges the NHS to resolve "difficult questions about hospital configuration" – that is, to shut units or even entire hospitals that are no longer viable – before the GP consortia start work in 2013. Lansley's new policy of "price competition", allowing hospitals to compete for patients, also poses a risk to standards of care, it adds. The report expresses concern that the reforms are being implemented at a time of spending restraint, suggesting it is "extraordinarily risky" to be restructuring the NHS when it also has to save £20bn by 2014-15.

The shadow health secretary, John Healey, said the report was a "comprehensive demolition job" and "a big red warning light ahead of the government's legislation."

The health department said that the NHS had to play its part in creating a system "that puts patients at the heart of everything it does, focuses relentlessly on improving healthcare outcomes and liberates professionals at every level to take decisions in the best interests of patients, rather than being micromanaged by politicians and civil servants."

It added: "The Health and Social Care Bill will provide a clear legislative framework to support that ambition."