MPs may not debate the government's controversial health reforms on the floor of the Commons despite an e-petition calling for them to be scrapped reaching 140,000 signatures, it was claimed on Friday.

Under the government's scheme, many signatories hope that by putting their name to an e-petition they can force MPs to debate the issue in the House of Commons.

But Labour MP Natascha Engel, chairwoman of the Backbench Business Committee, said the amount of time allocated by the government meant it was increasingly difficult to schedule debates on contentious issues such as NHS reform.

A number of rebel Liberal Democrats or dissenting Tory MPs would also have to back the Drop the Health Bill petition as her committee can only schedule debates with cross-party support, she said.

Engel's comments come as Labour leader Ed Miliband said the NHS would become the "defining issue" at the next election.

In a visit to the Royal Bolton Hospital, he said Prime Minister David Cameron had broken "solemn promises" at the last election to protect the NHS.

But despite the campaign against the NHS health reforms being at the top of the Labour leader's agenda, Ms Engel said she had "big concerns" there would not be enough time to schedule a debate in the Commons.

She said: "We have loads of different debates waiting for allocation. The Migration Watch debate still hasn't been scheduled.

"It's such a contentious issue and it's against the government's position and it's the government that allocates us time, so there is definitely a conflict of interests."

The government's website states any petition reaching 100,000 signatures "could be debated" in the House of Commons, but this is not guaranteed.

The Procedure Committee has said all e-petitions reaching six figures will trigger a hearing at least in Westminster Hall, a much smaller annexe where MPs can hold debates which do not require a vote.

The Bill has also attracted opposition from health professionals and patients' groups, while there have been reports of deep unease among Tory Cabinet ministers and even Number 10 insiders about the legislation.

On Monday, Downing Street dismissed suggestions that its author, Andrew Lansley, should be sacked as Health Secretary and insisted ministers were "fully behind" the reforms.

Cameron is expected shortly to launch a new offensive to "sell" the Bill to the public, saying at the weekend that he was "at one" with Lansley.

Today, Health minister Simon Burns hit back at Miliband's claims the Tories had reneged on their pre-election promises.

He said: "Ed Miliband can talk the NHS down all he likes, but the truth is that services are getting better all the time - with shorter waits since the election, 820,000 more people able to access an NHS dentist, 15,000 fewer administrators, 4,000 more doctors and 600 more midwives.

"Our plans deliver control for patients, power for doctors and nurses, and less bureaucracy. Labour still have no plan to help our NHS meet the challenges of the future. Their approach is simply one of cynical opportunism."