Nick Clegg told a meeting of his MPs in Westminster on Tuesday that he would now be "taking the lead" within government to rein in its programme of reform for the NHS.

The Liberal Democrat leader said he was determined to ensure changes were made to the health and social care bill, the clearest sign that he will personally negotiate with the health secretary, Andrew Lansley. A senior party source said that the Lib Dem leader had decided to "front up" the issue with the Conservatives.

Lansley's reforms to the NHS – handing over a majority of the healthcare budget to GPs for commissioning, and scrapping primary care trusts – have been opposed by some Conservative MPs and the British Medical Association, and 10 days ago Lib Dems voted at their spring party conference to ensure modifications were made to the bill. Then it was suggested that even though the Lib Dems had registered their discontent, there was little their leader would be able to do within government.

Now Clegg's aides say he will push for alterations to the bill, including beefing up the governance and accountability while minimising the financial risk of the new GP consortiums, and having some limits on the types of new private providers that can come into the system.

Clegg's closest colleagues – his parliamentary adviser Norman Lamb and the chief secretary to the Treasury Danny Alexander – have been newly charged with liasing with their backbench MPs and the wider party, to secure policy modifications. There is speculation that Clegg's opposition to the bill would delay it until beyond the Easter recess as officials work behind the scenes to reformulate a bill to the tastes of the Lib Dems.

However a source said Tuesday that it might not be necessary to delay the bill with changes being tabled in the House of Lords stage instead.

Clegg addressed his MPs hours after Lansley went before the commons health select committee to explain the progress of the bill.

He surprised some MPs when he admitted they were "still thinking through" what would happen should one of the newly empowered GP consortia go bust. Lansley said: "The responsibility lies with the NHS commissioning board. To identify and then intervene – they will have powers to take over responsibility or ask another hospital to take over." But he said there was still a debate about what would be the "trigger" for the NHS commissioning board to step in.

In an overture to those concerned about the reforms, and in a nod to the parliamentary push back from Clegg's team, Lansley said there would be "opportunities to clarify, if not improve, the bill as it goes through parliament."

Lansley insisted that the bill was compatible with EU law and that he was supported by legal advice. But when pressed five times by Labour MP Valerie Vaz to publish the exact legal advice, he appeared to refuse.

The level of confusion was such that the committee's chair Stephen Dorrell asked for a note of clarification on the issues of competition.

He also admitted that the government had not yet worked out the role of Monitor, the regulatory body that ensures fair play between GP consortia.

The Tory MP for Totnes, and a GP, Sarah Wollaston suggested that without clear legislated restrictions the "public interest" of Monitor was "clearly open to interpretation". Lansley committed to sending a written explanation on the regulatory body.

Lansley said the current debate was turning on an incorrect sum and that the size of the budget being handed to GPs was not £80bn as has been accepted but £60bn, owing to elements within the budget being redirected to areas like public health and regional specialised services which will not be channelled through GP consortia.