NHS trusts reported a third-quarter year-end deficit of £886 million, new data shows.
Figures from NHS Improvement mean the NHS in England currently has an overspend about £300 million higher than its target for the end of the financial year.
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 data suggests that winter had a negative impact on NHS finances, with lost income from cancelled operations and higher-than-expected spending.
The NHS Improvement report said that despite extra effort, "the current forecast deficit remains significantly higher than that planned. This is both unaffordable and unsustainable."
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.
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.
"Today's figures do show the considerable progress trusts have made this year to cut last year's record £2.45 billion deficit.
"We estimate the year-end deficit will be somewhere between £750 million and £900 million. However, this will still need clinical commissioning groups (CCGs) to support trusts where necessary, such as using the money saved from cancelled operations.
"But we shouldn't kid ourselves. The NHS' underlying financial position is not sustainable."