Hospitals in England ended last year with twice as big a deficit as expected, according to sources, in another illustration of the NHS’s fragile finances.

NHS Improvement (NHSI), the health service’s financial regulator, will reveal the overspend when it releases full details on Thursday of how the NHS performed in 2017-18. Sources close to the publication of the annual health check confirmed NHS trusts ended 2017-18 “about £1bn” in the red.

The likely overspend, double the £496m expected, will fuel claims that the government is underfunding the NHS, given the sharp increase in the number of people needing care.

It would show the health service has been unable to regain the spending discipline the Treasury demanded after years of steadily worsening finances. But critics will blame the deficit on the NHS experiencing the seventh successive year of a budget squeeze and hospitals having to staff thousands of extra beds as a result of the worst winter crisis in its history.

The deficit would be significantly higher than the £791m overspend in 2016-17, but less than the record £2.45bn the NHS ran up in 2015-16.

Jeremy Hunt, the health and social care secretary, is pressing Theresa May to honour her pledge of a long-term finding deal by giving the service a budget increase of between 3% and 4% a year at least until the end of the parliament in 2022.

Simon Stevens, the NHS England chief executive, wants a 10-year commitment to 4% annual rises. However, Philip Hammond, the chancellor, is privately warning that anything above 2.5% is unaffordable.

A report last week from the Institute for Fiscal Studies, the NHS Confederation and the Health Foundation warned people would have to pay more tax in order to give the NHS enough money in the years ahead to ensure it can provide care to an ageing population.

NHS finance experts claimed the health service’s deficit is much worse than the £1bn NHSI will admit to this week, because trusts also received £1.8bn from the sustainability and transformation fund, and £337m to help them cope with extra demand over winter.

Sally Gainsbury, a senior policy analyst at the Nuffield Trust, a thinktank, said: “NHS providers started the financial year 2017/18 with a £4bn black hole between their underlying costs and income that was deepened further over the year.

“So while hospitals and other NHS services did make efficiency savings over the year, the vast bulk of those savings were needed just to stop the black hole getting any deeper. Essentially, services are having to run to just stand still, or even move slightly backwards.

“The real underlying deficit is likely to remain very similar to where it was at the start of the year – at around £4bn, which is inevitable as long as we continue to systematically pay hospitals and other services less than the cost of actually delivering care.”

In March, the public accounts select committee said NHS finances “remain in a perilous state”.

“The NHS is still very much in survival mode, with budgets unable to keep pace with demand. The NHS has a long way to go before it is financially sustainable,” the committee said.

In February, NHSI disclosed that 107 of 136 hospital trusts providing acute care in England were in the red.

Niall Dickson, the chief executive of the NHS Confederation, said: “[Trusts] are at the end of their tether. It’s simply not realistic or reasonable to expect the NHS to go on delivering a comprehensive, universal service with inexorably rising demand and demonstrably inadequate funding.”

He urged ministers to abandon a short-term approach by which the NHS “lurches from budget to budget, with one futile bailout after another”.

