The NHS is facing a mounting financial crisis with more than half of all hospitals now in deficit and the service likely to end the year almost £1bn in the red.

Two sets of official figures later this week covering the NHS's performance in April, May and June will confirm a sudden, sharp and dramatic deterioration in its financial health between the first three months of 2014 and that quarter.

The pressure on services is so great and money so tight that, for the first time, a majority of hospitals ended that period in the red and accumulated a collective deficit approaching £500m.

NHS officials familiar with the situation say the speed with which financial troubles have deepened, and sheer number of NHS hospital trusts now affected, is causing unease at the Department of Health. Insiders say that figures due out on Friday are far worse than expected and likely to make "grim reading" for health secretary Jeremy Hunt. Hunt has already given hospitals more than £1bn of extra funding over the last year to help them cope with winter pressures and tackle a mounting backlog of operations, which recently reached a six-year high.

NHS leaders say privately that without an urgent bailout from the government, hospitals' overall deficit is likely to have increased further, and be approaching £1bn, by the time the NHS's financial year ends in March.

And as many as three-quarters of all England's hospitals could be in the red by then, they believe.

Hospital chief executives say the need to hire extra staff to maintain care standards in the wake of the Mid Staffs scandal, demand for A&E care, cuts in the fees they receive for treating patients and the NHS's £20bn savings drive has left them increasingly unable to balance their books.

One hospital boss said that the demand for care is so relentless, especially from older, frail patients, many of whom need to be admitted as an emergency, that "it's now like winter all year round". Last month one hospital saw a record number of patients turn up at its A&E unit on one day, despite it being summer and a lack of major illnesses such as flu.

Chief executives of hospitals describe the NHS's financial position as "a car crash" and "an absolute disaster" and warn that they can no longer provide services of the quality expected without incurring ever larger deficits.

Later this week Monitor, the NHS's financial regulator, will reveal that for the first time the semi-independent foundation trust hospitals it oversees – which are the NHS's best-run and strongest performing hospitals financially – posted a deficit for April, May and June rather than their usual surplus.

The 83 acute hospital trusts it regulates between them ended up more than £100m in the red. More than half lurched into deficit in that quarter, a big increase on the 34 such trusts, or 41%, which ended the 2013-14 financial year in deficit. Many trusts which usually make a surplus proved unable to do so amid the growing pressures, which also include the use of expensive agency and locum medical staff to plug gaps in rotas, especially nurses.

The steep decline in foundation trusts' financial performance – the sector overall made a surplus of £133m in the last financial year – is particularly significant, given their history of consistently sound financial management and profit-making since their creation in 2004.

However, the figures for the 62 NHS acute hospital trusts overseen by the NHS's Trust Development Authority (TDA) are worse. As with foundation trusts, the number of those traditionally weaker-performing trusts in deficit rose in April, May and June, from 24 (39%) in 2013-14 to more than 30, incurring a collective deficit of over £300m.

The hospital sector as a whole posted a deficit of £108m in 2013-14, the first time it had ended up in the red since the NHS's last cash crisis in 2005-06.

Foundation trusts have spent an estimated £1.2bn improving care, mainly on extra staff, in response to Robert Francis's Mid Staffs report last year and the review of NHS patient safety by Prof Don Berwick.

Monitor and the TDA both declined to comment before publishing their latest respective quarterly reports.

Labour blamed the coalition's radical restructuring of the NHS last year for the situation. "David Cameron chose to put NHS finances on this knife-edge when he wasted £3bn on a damaging reorganisation and clawed back a further £3bn to the Treasury in recent years. That is why the NHS finds itself in a such a fragile financial position," said Andrew Gwynne, the shadow health minister.

The Department of Health insisted that, despite the NHS's financial squeeze, all trusts must control their finances while also providing the best possible care.

"We've taken tough decisions to increase the NHS budget by £12.7bn over this parliament," said a spokeswoman.

"We understand some trusts are facing challenges because of increasing demand but they must have a tight financial grip and ensure they live within their means. Delivering high quality services and balancing the books must go hand in hand and we expect trusts to deliver this during the course of the financial year."