Pressure on Andrew Lansley and his NHS reforms was taken up another notch on Monday morning as the influential medical journal The Lancet comprehensively challenged the Tories' long-standing claim that productivity in the NHS is falling.

The latest attacks on the grounds for the Health and Social Care Bill come a day after David Cameron threw his support behind his beleaguered health secretary, saying he and Lansley were "at one".

But the PM"s support follows a difficult seven days for Lansley, during which Number 10 staff allegedly briefed against him and three Cabinet ministers told ConHome that they wanted the reforms to be dropped.

A study of quality of healthcare and mortality in the Lancet published on Monday morning suggests the popular consensus shared by many politicians - particularly Tory ones - is a myth.

The researcher, Nick Black, shows how many previous assumptions about productivity were based on incomplete or narrow interpretations of data. He writes that when the parameters for judging performance were drawn up in 2005, the Department of Health itself warned it was impossible to get the full picture from them.

Despite these warnings the Office of National Statistics kept dutifully pumping out figures showing a decline in productivity, the initial warnings about the reliability of the data were ignored, and over time the false assumptions were fuelled by "journalists seeking bad news", who failed to challenge the statements made by Tory politicians.

The report concludes: "By 2008, with the prospect of an imminent general election, repeated warnings from experts to treat the estimates cautiously were a forlorn hope."

Under new analysis in The Lancet report, productivity, patient outcomes and satisfaction in fact continued to increase right through to 2009. Yet in the same year Andrew Lansley told The Independent newspaper: "NHS spending has rightly increased. The tragedy is that costs went up and productivity went down. Labour has demonstrated that you can spend more to get less."

The Lancet report concludes: "We cannot prevent myths developing but we should remain vigilant, spot them as early as possible, and attempt to minimise the harm they can do in distorting understanding and misleading policy makers."

The Department of Health made no attempt to challenge the latest analysis, saying only: ""We have always been clear that productivity in the NHS needs to improve and are committed to better outcomes for patients across the country."

But Dr Richard Horton, Editor of The Lancet, said: “One of the reasons the current government gives for its radical and widely unpopular health reforms is the decade-long decline in productivity of the NHS. As Nick Black shows, this alleged decline is a myth. In fact, a fuller account of the evidence reveals likely substantial gains in productivity-gains in evidence-based practice, patient outcomes, and patient experiences.

"If the main reason for the Health and Social Care Bill is a lie, the upheavals it will produce are entirely unnecessary. This is further evidence to kill this damaging and dangerous Bill," Horton added.

On Monday the Health and Social Care Bill continues its passage through the House of Lords, with the prospect of peers making further amendments to the legislation.

On Sunday Simon Hughes, the deputy leader of the Lib Dems, said he thought Andrew Lansley should step aside in the second half of the current parliament.

His comments are likely to be taken by Lib Dem MPs as a sign to increase the pressure. The party grassroots have been lobbying their MPs to be more critical of the reforms, which appear to be opposed by practically every major body in the medical profession.