The health service in England faces a funding gap of more than £1.5 billion in the next financial year, figures suggest.
The NHS's secondary healthcare budget in England, which includes money for hospital and mental health services, is £78 billion, but analysis of figures from the health regulator Monitor suggest that in 2015/16 there will be a funding gap of £5.1 billion.
But even after all realistic efficiency savings are made there will still be a deficit of £1.6bn for 2015/16, according to the BBC analysis.
Health economists have said in the past that the NHS is facing a "looming financial crisis".
The King's Fund has previously warned of a bleak financial outlook for the NHS in 2015/16.
The influential think tank said that a "significant" boost in funding is needed for the NHS to prevent an impending crash.
Health officials said that the NHS is "on track" to make £20 billion in efficiency savings it was tasked with making.
A Department of Health spokesman said: "The NHS is on track to make £20 billion savings this Parliament and we are confident that it will continue to make the savings necessary which will be reinvested to front line care to meet the rising demands of our ageing population."
But Labour said that the health service's finances are on a "knife-edge" as a result of the controversial NHS reforms.
Shadow health secretary Andy Burnham said: "David Cameron chose to put NHS finances on this knife-edge when he wasted £3 billion on a damaging re-organisation and clawed back a further £3 billion to the Treasury in recent years. That is why the NHS is in a such a fragile financial position."
Christina McAnea, head of health at the union Unison, said: "The prospect of a further £2 billion cut in the NHS budget is a real blow for patients and staff.
"This Government has wasted precious billions on a massive and unnecessary top-down reorganisation of the NHS.
"The Government can't expect staff to plug this hole in NHS funding by cutting pay and conditions even further. Unison members are already angry over the Government's decision to deny 60% of NHS staff a pay increase. And we will ballot our members for strike action.
"Starving NHS budgets further will have a significant and noticeable impact on patient care, staff training and hospital infrastructure.
"The NHS is already under strain because of rising demand and the high cost of expensive drugs and treatments."
NHS Confederation's chief operating officer Matt Tee said: "There is no doubt that the health service is experiencing some of the most pressured times of its existence so far.
"What we really need is for the NHS to have some certainty about the funding it will get in future years, so health service leaders can plan most effectively with their populations how to deliver the right services, in the right places, for the best value.
"We are calling on politicians to commit to a 'decade deal', a 10-year funding settlement for the NHS, and pledging in their 2015 election manifestos to protect NHS funding in real terms for the next 10 years, at a minimum."
Prime Minister David Cameron's official spokesman said: "It is this Government that has protected spending in the NHS. As part of that, there is a programme of efficiencies of £20 billion.
"Those efficiencies have been delivered in every year so far and we are very confident they will be next year as well."
Mr Cameron told MPs at Prime Minister's Questions in the House of Commons: "The estimates being made today are being made on the basis that we have set challenges for the NHS in terms of making efficiencies.
"After four years in Government, they have met those efficiency challenges every single year under this Government and that money has been ploughed back into better patient care in the NHS."