Cameron promises 'significant and substantial changes' to health and social care bill as Ed Miliband accuses him of 'dumping on' health secretary Andrew Lansley

David Cameron has promised "significant and substantial changes" to the government's plans to reform the health service amid accusations that ministers are conducting a "sham" consultation exercise.

The prime minister became embroiled in a row with the Labour leader, Ed Miliband, at prime minister's questions over treatment waiting times and plans by the health secretary, Andrew Lansley, to revamp the National Health Service, which have sparked widespread opposition from staff.

The two leaders traded insults as Miliband ramped up the pressure on Cameron by linking him personally with the NHS reform plans.

The Labour leader read out a quote from Cameron last month, in which he claimed he had been involved in designing the reforms "way back into opposition with Andrew Lansley".

"Can he therefore confirm that the NHS plans are not Andrew Lansley's fault; they're his?" asked Miliband, before accusing him of "dumping" on a minister rather than taking responsibility himself – a practice Miliband claimed was "becoming a pattern".

"This morning in the papers we see the universities minister [David Willetts] being dumped on for his tuition fees policy, we see the schools secretary [Michael Gove] being dumped on for his free schools policy and the poor deputy prime minister [Nick Clegg], he just gets dumped on every day of the week."

He said the decision to throw open a "listening exercise" last month, with the legislation for the reforms already making its passage through parliament, was proof that Cameron thought "something has gone wrong".

But he went on to claim that the exercise was "nothing more than a sham" in light of the fact that the NHS chief executive, Sir David Nicholson, had told health managers to carry on implementing the reforms after the "so called pause" in the legislation had begun.

Cameron insisted the government was determined to get the reforms "right".

"I can absolutely guarantee there will be significant and substantial changes to the reforms because we want to get them right, because we want to guarantee an NHS, free at the point of use, available on need rather than ability to pay and unlike the party opposite, which is cutting the NHS in Wales, this government will put more money in the NHS."

The prime minister cited a letter signed by 42 GPs that hailed Lansley's health reforms as "good for patients".

The letter, published today in the Daily Telegraph called on the government to press ahead with the health and social care bill.

The 42 signatories, all family doctors, who together lead 1,100 practices across England and who are all heads of recently-formed GPs' consortiums, said that ditching the reforms would be a mistake.

They insisted the reforms would benefit the most elderly, infirm and vulnerable people in society.

But it emerged almost three weeks ago that Jonathan Munday, a former Tory councillor who chairs the board of the Victoria Commissioning Consortium, had emailed GP colleagues urging them to back his reforms by signing the letter of support to the prime minister that went out today.

Miliband told Cameron he should be "embarrassed" by the assessment by the Royal College of General Practitioners, which represented thousands of GPs, and has warned that the government's plans would cause "irreparable damage to the core values of the NHS".

The prime minister challenged Miliband to deal "with the substance of the reform" since he agreed that "no change is not an option".

"He should be seriously engaging in how we make sure we have a strong NHS for all our people in the future. Instead we have an empty opposition, which got him nowhere last week."

Miliband used his first question to ask Cameron to rate his handling of the NHS after a year in office before pointing to figures published today which show waiting times for diagnosis have risen : "Over 10,000 people waiting to get their tests, that's three times the number it was a year ago."

Cameron instead seized on a previous claim Miliband made two weeks ago about rising waiting times for inpatients and outpatients, which the prime minister said had turned out to be wrong, and told the Labour leader he should have the "guts" to admit he had been wrong.

But the Labour leader insisted "waiting times are rising" and said Cameron had refused to take responsibility for the NHS reforms.

The NHS surfaced again later in prime minister's question as Cameron told MPs that the government would consider terminating its contract with a private firm to overhaul the NHS's computer system.

He faced calls from Richard Bacon, the Conservative MP for South Norfolk, to scrap the new system and save a further £4.7bn, which he said could be better spent directly on patients. The NHS computer programme would "never deliver its early promise", he added.

Cameron said the IT systems inherited from Labour were "poor value for money", with the £12bn centralised records system for 50 million patients in England taking four years longer than planned to implement.

He told the Commons: "Since coming into government we have reviewed the projects with the intention of making the best of what we have inherited.

"In part, as a result of our work, the government has cut £1.3bn from the cost of the National Programme for IT in the NHS, including planned savings of at least £500m from Computer Sciences Corporation."

He added: "There are no plans to sign any new contract with Computer Sciences Corporation until the National Audit Office report has been reviewed and until the public accounts committee meetings and the Major Projects Authority reviews have taken place.

"The Department of Health and Cabinet Office will examine all the available options under the current contract, including the option of terminating some or all of the contracts."