The Tories and Liberal Democrats are facing a fresh clash over the government's NHS reforms after Nick Clegg encouraged his MPs to put "probing questions" to ministers when the bill returns to the Commons on Tuesday.

In a two-hour meeting with his parliamentary party on Monday night, the deputy prime minister held out the possibility that he will accept amendments to the heath and social care bill when it moves to the House of Lords later this month.

Clegg's move means that Lady Williams could be backed by Liberal Democrat ministers if she attempts to amend the bill to guarantee that the health secretary has a legal duty to deliver a comprehensive health service free at the point of need.

But a source at the Department of Health indicated last night that Andrew Lansley, the health secretary – who has already amended the bill after the government's "listening exercise" – would not accept fresh amendments on this point.

The source said: "Our view is that the legislation is watertight on the secretary of state's obligation to ensure there is an NHS available to all. That was always our view. But we amended the legislation to reassure those who were not sure."

Clegg said earlier in the day that he accepted the view that there was no need for fresh amendments on this issue.

In a speech on schools in south-west London, he said: "Let me be absolutely clear. There is nothing, nothing, nothing in any of the government's plans which in anyway threaten the basic founding principles of the NHS...There is no question, legally or politically, of the secretary of state under these new arrangements being somehow able to wash his or her hands of the NHS."

But at Monday night's meeting of the Lib Dem parliamentary party, Clegg admitted that ministers still had to work hard to clarify the bill for MPs and peers with concerns.

Paul Burstow, the Lib Dem health minister, is to offer further briefings to MPs and peers who will also be invited to meet officials at the department of health.

All sides accept that it is too late to table further amendments on the NHS reforms when the bill is debated by MPs at report stage on Tuesday and Wednesday and at third reading on Wednesday. But Lib Dem MPs have been encouraged to put "probing questions" to ministers for possible amendments that will be tabled in the House of Lords.

One Lib Dem source said: "We hope that we will not need to amend the bill further. But we may have to." Another Lib Dem source said: "There will be robust interventions in the debate."

Lib Dem whips believe that the overwhelming number of MPs will support the amended bill. But Andrew George, the Lib Dem MP for St Ives, said he would rebel.

The battle within the Lib Dem ranks was exposed in leaked emails, in which grassroots members of the party vented their anger at the leadership.

Jeremy Sanders of Huddersfield Liberal Democrats wrote an email to John Pugh, the Lib Dem backbench health committee chairman, saying: "We can try to get improvements to the details, but none of these changes are going to alter the basic fact that the legislation is based on the assumption that what the NHS needs is a system based on private sector involvement, free market competition and internal markets.

"Quite honestly, if our MPs are willing to go along with this, what exactly won't they be willing to support?" In the same batch of emails obtained by the Guardian, Robert Hutchison, a Lib Dem councillor in Winchester, tells Pugh "in my view is that if Lib Dem MPs vote for the bill this week — without further major amendments — it will damage the NHS and damage the party".

Charles West, one of the key party activists on the NHS, has written to party members to back an appeal against the decision by the Lib Dem conference committee to not debate the health bill at the forthcoming party conference.

He wrote: "I have therefore written a letter of appeal to the Federal Conference Committee against their narrow decision not to take the motion that I and over 100 conference reps submitted in June, and in case that appeal fails we are submitting an emergency motion which will achieve the same ends".

Last month Andrew George, the Lib Dem rebel on the health bill, emailed Lib Dem activists with a blunt message: "Of course I'll try to influence colleagues but some are still basking in the synthetic afterglow of the post-pause Bill revision, perhaps having duped themselves that it's 'job done'! People need to wake up to the fact that we can say what we like at conference, but the MPs' main chance to influence would already have passed!"

Labour twisted the knife into the Lib Dems with the party's health spokesman John Healey arguing that Nick Clegg's claim that he had met 11 out of the 13 changes demanded by his party's spring conference resolution was "wrong". "He's failed on seven and fallen short on six," he said.

Baroness Thornton, Labour's spokesperson in the Lords, warned that the lack of scrutiny in the Commons — where 1,000 amendments mean just 40 seconds of parliamentary time to consider each one — could see the bill be put into a specialist committee to examine whether there is enough time to debate the reforms.

Writing in the Guardian, Tory MP Sarah Wollaston, a former GP who had criticised the health bill, says now is the time to back the coalition's plans as "the structural changes to the NHS have passed the point of no return". She argues instead that the bill needs to be amended to ensure that membership of the NHS National Commissioning Board, a quango with £60bn to spend, is fairly and openly discussed.

Meanwhile the King's Fund, a leading health thinktank, said that closing hospitals should be made easier by removing politicians from the decision-making process because they are inclined to be swayed by public opinion not clinical need. While the current system is "complex and bureaucratic", the fund warns the proposed reforms would make the situation worse with no regional overview and three extra layers of bureaucracy – which could all disagree with one another. Faced with making £20bn in "improvement savings" over the next four years, the fund says there are "a number of NHS trusts for whom reconfiguration is expected to be necessary to achieve a sustainable financial position".