Government claims that competition has improved quality in the NHS and has saved lives are based on flawed research, experts say.
As ministers prepared to defend Andrew Lansley's health reforms, which reach the House of Lords this week, academics at the University of London have attacked a key pillar of the Health and Social Care Bill.
It came as the Cabinet and the Department of Health were put on alert to prevent a "peers revolt" that could lead to parts of the Bill being referred to committee – fatally stalling the legislation.
Liberal Democrats, led by Dame Shirley Williams, were alarmed that the duty of the Secretary of State to provide a free, universal and comprehensive health service was being weakened. Ministers are expected to offer concessions to reassure them.
Fears that the Bill will be a Trojan horse for the private sector are also fuelling protest. Some 2,000 demonstrators blocked Westminster Bridge yesterday during a demonstration organised by UK Uncut against "the wholesale privatisation of the NHS".
Andy Burnham, the shadow Health Secretary, said the legislation would turn the NHS into a free market and called on Mr Lansley to withdraw the Bill.
The latest criticism of the Bill, by Professor Allyson Pollock of Queen Mary, University of London, and her colleagues, challenges the evidence on which the reforms are based. Writing in The Lancet, she said that a study used by David Cameron to back the case for more competition in the NHS was littered with errors.
That study, by researchers at the London School of Economics, suggested that mortality rates for heart attack patients were lower in cases where more hospitals were within travelling distance of the patient's GP surgery. It also looked at outcomes from routine surgery on hernias, hips, knees and cataracts and concluded that greater choice of hospital led to better results for heart attacks.
Professor Pollock said the study offered no explanation of why the availability of choice for the routine procedures should have had any effect on the mortality of heart attack patients. Heart attack is a medical emergency and most patients have no choice about where they are treated. She added: "Our examination of this research reveals it to be fundamentally flawed. The paper simply doesn't prove either cause or effect between patient choice and death rates."