Nine out of 10 hospital bosses in England fear understaffing across the service has become so severe that patients’ health could be damaged. In addition, almost six in 10 (58%) believe this winter will be the toughest yet for the service.

The views expressed by senior NHS figures on Tuesday will heighten the anxiety in Conservative ranks that the health service’s growing problems risk derailing the party’s campaign in an election members hoped would be dominated by Brexit.

The Labour party is seeking to capitalise on public dissatisfaction over delays in accessing treatment and the increasingly visible gaps in staffing.

In a further sign of Tory concern, ministers have agreed an extraordinary deal for the NHS to pay doctors’ pension tax bills this year, which could cost hundreds of millions of pounds.

The scheme is aimed at halting the sharp recent increase in doctors working fewer shifts in order to avoid being hit with unexpected tax bills of up to £100,000. The trend has forced hospitals to cancel thousands of operating lists and outpatient clinics, while further delaying patients’ access to care and exacerbating staff shortages.

Ministers hope doctors in England – the only country the incentive will apply in – will see it as a green light to resume extra shifts before winter pressures ramp up on the NHS, without having to worry that they will be heavily penalised months later.

However, the deal immediately triggered claims that it has been agreed between ministers and NHS England in defiance of “purdah” rules that stipulate that governments must not undertake changes of policy during an election campaign.

It is being presented as an “operational decision” by NHS England, but was signed off – and some believe instigated – by the Treasury, Cabinet Office and the Department of Health and Social Care.

A senior medical source involved in brokering the unprecedented “stopgap” policy suggested it came about because ministers were “desperate” to avoid fewer shifts by doctors compounding hospitals’ struggles this winter.

The source said: “They have so massively breached purdah regulations it’s unbelievable. This isn’t an operational matter. This is policy. It’s outrageous, because purdah rules say that you can’t announce a change of policy during an election.”

The 131 chief executives, chairs and directors of NHS trusts in England expressed their serious concern about the deteriorating state of the service in a survey conducted by the NHS Confederation.

The findings came days after the latest official figures showed that hospitals’ performance against key waiting times for A&E care, cancer treatment and planned operations had fallen to its worst ever level. However, many service chiefs told the confederation that delays will get even longer when the cold weather creates extra demand for care.

“There is real concern among NHS leaders as winter approaches and this year looks particularly challenging,” said Niall Dickson, the chief executive of the confederation, which represents most NHS bodies, including hospital trusts, in England.

“Health leaders are deeply concerned about its ability to cope with demand, despite frontline staff treating more patients than ever.

“There is the very real prospect of gaps in clinical shifts and patients not receiving the quality of care they need because NHS trusts do not have the staff they need.”

“Despite doing everything within their power, 90% of health leaders we surveyed said that understaffing was putting patients at risk.

“We have 100,000 clinical vacancies [in England] and the prospect of ever-rising demand unless we face up to the scale of the challenge,” added Dickson.

Last week’s figures showed that one in four people who attend a hospital-based A&E are waiting more than four hours to be dealt with, record numbers are having to wait on a trolley while they are found a bed and seven of the eight clinically vital cancer treatment targets are being missed.

Dickson added that, even if the next government provided more money to tackle widespread staff shortages, it would take time to reduce the high vacancy rates that are common in many hospitals. The NHS is short of about 43,000 nurses and almost 10,000 doctors as well as paramedics and other health professionals.

He warned political parties not to raise voters’ expectations unreasonably in the run-up to polling on 12 December about how quickly the NHS can get back on track.

“More investment is needed but even with that this is a system that will take time to turn around and the electorate must not be fed with overpromises over the coming weeks,” he said.

The King’s Fund voiced concern at the results of the research. “Amidst the political rhetoric of the general election campaign, these findings underline the stark reality facing patients across the country who are struggling to access NHS services,” said Sally Warren, the thinktank’s director of policy.

“Workforce shortages are already having a direct impact on the quality of people’s care, with national patient surveys repeatedly highlighting difficulties for patients accessing NHS services and performance against key waiting time targets at their worst in over a decade.

“These NHS leaders are correct – without urgent action patient safety will be at risk.”

The confederation’s survey of 131 hospital bosses also found that:

76% say staff shortages are the NHS’s most pressing problem.

83% say the dispute over senior doctors’ pensions is making understaffing even worse.

69% say doctors deciding to work fewer hours is damaging patient care.