Winter pressures have caused the NHS to overspend as hospitals and other services have struggled to keep up with demand in England, finance chiefs say.
A deficit of nearly £900m was racked up by NHS trusts in the first nine months of the 2016-17 financial year.
It comes despite the health service being given extra money to help it get on top of its finances after the record £2.45bn overspend in 2015-16.
Hospitals were seeing more patients than budgeted for, they reported.
They also said problems discharging patients because of a lack of community services had cost them, said the regulator, NHS Improvement.
The figures for April to December cover ambulances, mental health units and community services as well as hospitals - although most of the deficit has been accrued by the latter.
Between them they account for £80bn of fund, about two-thirds of the health budget, because spending on GPs, training, drugs and public health are accounted for separately.
NHS Improvement, which released the accounts, said it had been a "challenging winter".
Waiting times have reached their worst-ever levels in A&E, while nine out of 10 hospitals have spent the winter months overcrowded with unsafe numbers of patients on wards.
NHS Improvement chief executive Jim Mackey said it was proving to be "extremely challenging times".
But the regulator predicted the deficit could be cut slightly by the end of the financial year in April to between £750m and £850m - but still above the £580m figure suggested earlier in the year.
Some 135 out of 238 trusts had racked up a deficit in the nine months between April and December.
The total deficit when taking into account surpluses was £886m - less than half the figure at this point last year.
But the improvement has only been achieved because of a special one-off £1.8bn fund this year to help hospitals plug the gap.
Chris Hopson, chief executive of NHS Providers, which represents trusts, described the latest figures as worrying.
He said trusts were expected to operate with a "wafer-thin" margin for error.
"We shouldn't kid ourselves. The NHS's underlying financial position is not sustainable," he added.
The NHS is in the middle of the tightest financial settlement since it was created. Since 2010 the budget has been rising by a little more than 1% on average compared to more than 4% during the rest of its history.