The deputy prime minister, Nick Clegg, has signalled that a key deadline set by the health secretary, Andrew Lansley, for a switch to GP-based commissioning will not be implemented unless GPs can prove they are ready to take the responsibility.

In a sign that the government is prepared to make a key concession on Lansley's reforms amid tensions within the coalition, Clegg said "alternative arrangements" would be made if GP practices were not ready by April 2013, the date set by the health secretary.

His comments came as Dr Peter Carter, the chief executive of the Royal College of Nursing (RCN), told delegates at the union's annual conference in Liverpool that the reforms could "well turn out to be the biggest disaster in the history of our public services" if unions and other organisations were not listened to.

He warned that the sheer "size, importance and reach" of the changes could not be overstated.

Carter said nurses were struggling because of the government's two-year pay freeze, rising costs and increasing workloads and "never before" had so many nurses talked to him about the possibility of industrial action.

Speaking to journalists afterwards, he said industrial action was "a long, long way away" and a process would need to be gone through, including balloting members.

He said nurses "would not damage patient care" by simply walking out of hospital wards. Options for action could include work to rule, such as only working contracted hours and taking all mealtime breaks.

Clegg revealed a more flexible approach as he sought to play down a threat from one of his closest allies to quit over controversial NHS reforms.

Norman Lamb, a government whip who serves as one of the deputy prime minister's senior advisers, said his party's MPs and peers would be unable to support the health and social care bill if their concerns were ignored.

Among the demands laid out by Lamb, a former Lib Dem health spokesman, was a call for a 2013 deadline for GP-based commissioning consortiums to be scrapped in favour of an "evolution, not revolution" approach, with GPs having to opt into consortiums and the retention of a cluster of primary care trusts to observe the "performance management" of GPs.

Clegg appeared to bow to one of the demands in an interview with BBC Radio 4's Today programme.

He said ministers "have to get it right" on NHS reforms, saying it was a "good thing" that the government would be listening to concerns over the shake-up in the next two months and was willing to "change things where necessary".

While he stressed that there were "some virtues and some drawbacks" to calls for GPs to opt into consortiums because of the "uneven" approach this could create in the NHS, he made it clear that the 2013 deadline imposed by Lansley could be relaxed.

Clegg said GP practices would have to meet a "whole series of tests" before being allowed to take on commissioning functions.

"I have been very clear we are not going to allow GP consortia who are not ready to take on these commissioning functions if they are not ready by the 2013 deadline," he said.

"You clearly set in motion a whole series of tests to make sure they are able to take on those responsibilities.

"If they are not, they will not, and then alternative arrangements would need to be made. Asked whether the government would reconsider the plan to scrap primary care trusts, Clegg said there was no point having a "pause" in the legislation unless there was a proper rethink. "I think it's a good thing that we are listening," he said.

He sought to play down party divisions by insisting that neither ministers nor his own party wanted to "reopen the Pandora's box of the basic design" of the bill – giving GPs more financial responsibility, reducing bureaucracy and giving local authorities a greater say in the way the health systems works.

While NHS reforms were necessary, "the devil lies in the detail" of the legislation, Clegg said.

"Everybody agrees it is right to put more financial responsibility in the hands of GPs who know the patients best, but how you do that ... the devil lies in the detail," he added.

"The details of how you make these principles work in practice are of course things we want to get right.

"I couldn't agree more with Norman – we have to get this right. The NHS is too precious, to me and everyone else who relies on it in the country, to not get the principle translated properly into practice."

Clegg rejected the suggestion that the Lib Dems were now portrayed as a party less committed to the idea of public services as a "complete caricature".

"It was Labour that crow-barred private sector providers – through so-called independent treatment centres – by rigging contracts with the private sector, which very deliberately, subconsciously undercut the NHS," he said.

"We are outlawing that. We are saying there has to be a level playing field with voluntary groups, the NHS and other providers and we are actually saying there should be no price competition whatsoever. In other words, we are providing a more open approach."

A series of "listening events" are to take place across the country over the coming weeks as Clegg, David Cameron and Lansley seek to demonstrate that they are taking criticism and advice on board.