Ministers could face a deepening series of problems in the NHS, including cancelled operations, rising waiting lists and worsening financial problems, a former adviser to David Cameron on the NHS has warned.

Prof Chris Ham, chief executive of the King's Fund health thinktank, said rising demand for healthcare and tight finances in the NHS meant it was likely to come under intolerable pressure as a result.

The state of the service, and the government's response to a new set of emerging difficulties, could see the NHS emerge as a major vote loser for the coalition at the 2015 general election, said Ham.

He was a member of Downing Street's now-disbanded "kitchen cabinet" of health experts formed in 2011 after the coalition's radical shakeup of the NHS in England proved to be highly unpopular.

"We know from the data on A&E that it is heading in that direction [of intolerable pressure]," Ham said. "The government will do very well to navigate the next two winters through to the election without the NHS being back in the headlines for all the wrong reasons.

"We could end up with patients on waiting lists having their operations cancelled because hospitals are forced to use their beds for patients coming in as emergencies, hospitals failing to meet the four-hour A&E target, difficulties treating patients within the required 18 weeks, ambulances waiting outside hospitals with patients inside them, and delayed transfers reaching such a level that hospital beds are full up and the whole system is under intolerable pressure."

Ham emphasised the potential political danger for the coalition: "For the next 18 months before the election, the NHS will become a much more salient and visible issue for the public and politicians. If the NHS isn't able to cope with the growing pressures it is under, the public will be influenced in their voting decisions by their perception that the NHS hasn't been well-managed by the coalition government.

"If I was in the Department of Health or Downing Street and looking across the whole of government policy and issues that will be important in the runup to 2015, nothing would worry me more and keep me awake at night than the state of the NHS."

Two senior NHS figures have also issued dramatic warnings about looming problems.

Stephen Thornton, the deputy chair of the NHS regulator Monitor, said the growing number of NHS trusts in financial trouble meant "we are heading for a disaster", with a number of hospitals at risk of ending up deep in the red.

Small and medium-sized district general hospitals are going to struggle to balance their books in the next few years, he said, and will find it hard to deliver their share of the NHS's £20bn efficiency drive next year.

Chris Hopson, chief executive of the Foundation Trust Network (FTN), endorsed Thornton's fears. He warned that there were "a number of clinically and financially unsustainable trusts" and some could go the same way as the South London Healthcare Trust, which collapsed in the summer of 2012 with debts of more than £150m. It was taken over by a special administrator and its hospitals have been taken over by other trusts.

Monitor, which regulates foundation trusts' financial affairs, recently indicated that it was preparing for as many as 15 to 20 hospital trusts to fall into a "failure regime" over the next five years.

Hopson warned that patients' lives could be lost because an urgently needed radical reorganisation of hospital services was being delayed because the competition authorities prioritised their own "esoteric" rules over the need for failing trusts to merge or lose some of their key services to other trusts nearby.

"All I can see is a very large train hitting a very large buffer" if hospitals are not allowed to stage takeovers, he said. A combination of tough NHS finances and the need to make 4% annual budget savings, deliver better clinical outcomes and guarantee foolproof patient safety in light of the report into the Mid Staffs care scandal could prove too much for some trusts, Hopson said.

Up to 30 trusts are unlikely to achieve foundation trust status, as the government has demanded, and need to partner with another NHS provider in order to survive, according to the NHS Trust Development Authority, which oversees them.