NHS foundation trusts have been ordered to "leave no stone unturned" as they try to save money to turn their huge deficit around.
In a letter, the chief executive of regulator Monitor, David Bennett, asked all trusts to review their funding plans, including such areas as ensuring vacancies are filled only where essential. He said the plans in place are not likely to balance the books.
There are 152 trusts with foundation status in England - seen as a hallmark of excellence in the health service - but in May it emerged that they reported a £349 million deficit for the last financial year - 35 times more than the £10 million planned.
It meant that the total deficit for trusts was £822 million.
A huge over-reliance on contract and agency staff was the main reason behind the overspend.
In the letter, Dr Bennett said their plans for this financial year are "quite simply unaffordable".
"As you know, the NHS is facing an almost unprecedented financial challenge this year," he said.
"Current plans are quite simply unaffordable. As I have said before, if we are to do the best we can for patients we must leave no stone unturned in our collective efforts to make the money we have go as far as possible.
"We are already reviewing and challenging the plans of the 46 foundation trusts with the biggest deficits.
"However, it is clear that this process will not close the funding gap and so we need all providers - even those planning for a surplus this year - to look again at their plans to see what more can be done."
He asked trusts to ensure that the existing acute inpatient safe staffing guidance "has been adopted in a proportionate and appropriate way" and that rosters are rigorously managed to deploy staff efficiently across all required shifts including evenings and weekends.
Janet Davies, chief executive and general secretary of the Royal College of Nursing (RCN), warned there should be no compromising on safe staffing levels, which could put patients at risk.
"Staffing levels are either safe or they are not, and this must be decided based on patient need, using safe staffing guidance," she said.
"If staffing levels are decided by accountants rather than clinical staff, patient care will suffer.
"It is also unclear what constitutes a non-essential job in an NHS trust. If you get rid of support staff, their work does not disappear. Instead, it will mean frontline staff picking up extra paperwork and spending less time with patients. This is a false economy.
"One of the main reasons for the growing NHS deficits is the increase in agency spending. A long-term solution is needed, training enough nurses so trusts can affordably provide safe staffing levels. Short-term cuts will prove more costly in the long run, both to trust finances and patients."
Dr Mark Porter, the British Medical Association's (BMA) council chair, said the letter raised further questions about how the NHS would be able to afford to become a "truly seven-day service", as proposed by the Government.
"Despite repeated calls, the Government's refusal to say how they will fund and staff them means that we and the public are no closer to finding out the detail," he said.
"It is simply wrong to expect doctors, nurses and other NHS staff already delivering weekend care to deliver more without the extra resource urgently needed.
"We already know that two-thirds of the public don't think the NHS can afford seven-day services. Today's news will do nothing to alleviate their or doctors' concerns."
Shadow health secretary Andy Burnham said the latest development was a "sign of a serious deterioration in NHS finances".
"It suggests that the financial crisis in the NHS is threatening to spiral out of control and hit standards of patient care," he added.
"Morale in the NHS is already at an all-time low and doctors have lost confidence in the Health Secretary.
"It will raise further questions about how the Government can possibly fulfil commitments on a seven-day NHS without the money to back it up."
The provider sector has forecast a total deficit of more than £2 billion for 2015-16, according to specialist resource Health Service Journal.