Video report by ITV News Correspondent Paul Davies
NHS trusts overspent by more than £300 million over the last financial year, new figures show.
Figures from NHS Improvement show the NHS in England reported a third-quarter year-end deficit of £886 million.
I can no longer guarantee patient safety.
The target is for the NHS to end the financial year £580 million in deficit.
The year-end forecast is now £873 million.
NHS Providers expressed concern that the latest figures rely heavily on one-off savings that cannot be made in the future.
The size of the deficit in NHS trusts in England
The financial year deficit target
Despite the overspend in a bid to meet the increase in demand, a Senior A&E Consultant has told ITV News that the pressure the NHS has been under this winter is "the worst" he has ever known.
He added: "For the first time ever I've had to phone the executive on-call to say I can no longer guarantee patient safety.
"This is down to us being completely overwhelmed."
NHS Providers chief executive Chris Hopson said: "Despite doing everything they possibly can, NHS trusts are £300 million behind the target of reducing the provider sector deficit to £580 million by the end of March.
"This is largely because of winter pressures. Trusts spent more than they planned and they lost income from cancelled operations - both were needed to create the extra bed capacity to meet record emergency winter demand.
"This shows the danger of planning with no margin for unexpected extra demand.
"We can't expect to run NHS finances on wafer-thin margins year after year and keep getting away with it. The NHS' underlying financial position is not sustainable."
A senior A&E Consultant from a city hospital tells ITV News of the pressures the NHS is under.
The figures cover hospitals, ambulances, mental health units and community services - although most of the deficit was in hospitals.
A survey of 99 NHS finance directors released by NHS Providers shows that while most trusts were on or ahead of plan, just over a quarter (27%) reported deteriorating finances.
Finance directors blamed a rise in A&E attendances and hospital admissions of 3.5% against a plan of 2%.
Two-thirds of trusts said they can only meet financial targets this year as a result of one-off savings - estimated to be as much as £1 billion - which may not be achievable next year and beyond.
The sum owed by NHS trusts is down from the record overspend of £2.45 billion in 2015/16.
In numbers: How is the NHS in England coping financially?
The number of trusts out of 238 which were in deficit, 44 fewer than the same period last year
Between October and December, A&Es saw 200,000 more patients than the previous year
The increase in the number of patients requiring major further in-hospital treatment
The locum and agency spend in December - the lowest monthly amount since October 2015
The Department of Health said: "This analysis is inaccurate. The figures come from two different time periods when the way of counting beds was different, and so they aren't comparable.
"Our hospitals are busier than ever but thanks to the hard work of staff, our performances are still amongst the best in the world."