Senior doctors in the NHS are reducing their hours, turning down extra work and even retiring early to avoid being hit with huge tax bills on their pensions, a report reveals.

The findings have prompted concern that waits for treatment will grow longer, patients will receive inferior care and understaffing will worsen because hospitals have too few doctors available.

Nearly 20% of consultants have cut the number of hours they work and 42% have reduced the amount of extra shifts they do, faced with the threat of receiving bills that can top £50,000 in a year, according to the research commissioned by NHS Employers.

Large minorities of consultants are considering taking early retirement (33%), reducing their hours (44%) or cutting the amount of extra work they take on (40%) because of the problem, a result of changes to the NHS pension tax regime in 2016.

As many as one in four senior doctors may quit the service altogether, according to the survey of 2,500 consultants in England earning more than £60,000 a year. Some doctors have been forced to remortgage their homes to pay hefty tax bills arising from their pension contributions while others are doing more work in the private sector because earnings there do not affect their NHS pensions.

The findings emerged as representatives of the UK’s 240,000 doctors at the British Medical Association’s annual conference on Monday claimed the “punitive and ill-conceived” charges posed “a direct threat to patient care” and represented “an existential crisis” for the NHS.

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One consultant quoted in the report said: “I am not alone in this and I see a huge number of my colleagues bewildered, disillusioned and forced to take steps that will weigh heavily on their conscience as the only way out of this punitive taxation is to reduce our work, leading to a disastrous compromise in patient care.”

NHS Employers, which represents England’s 240 health trusts, commissioned the research to understand the impact of the bills. The findings vindicate warnings from the British Medical Association (BMA), the doctors’ union, that pension tax charges, especially involving the annual allowance, are driving doctors out of the NHS workforce and offering them a “perverse incentive” to do less work for the service.

“It is clear that patients are facing the prospect of fewer senior doctors on their wards and in their hospitals providing care. There are already widespread workforce shortages across the NHS that are resulting in cancelled operations, long waiting lists and all-year-round pressures across the health service,” said Dr Rob Harwood, the chair of the BMA’s consultants committee.

“This is another blow to the NHS workforce that it can ill afford,” said Harwood, who demanded urgent government action.

The situation recently forced Barking, Havering and Redbridge hospital trust in north-east London to scrap plans to clear its backlog of patients awaiting care. It intended to put on extra surgery sessions to treat patients with gastroenterological conditions who had been waiting more than a year. But it had to cancel the clinics as consultants declined to do the work because their extra earnings could have affected their pension tax liability, the Health Service Journal reported.

Any consultant earning more than £110,000 a year is at risk of receiving a bill because the tax-free pension allowance of those earning above that was cut from £40,000 to £10,000.

The maximum consultant’s salary is £105,000 for doing 10 “programmed activities” – four-hour sessions of work – a week. But many earn more than that by doing extra work, receiving clinical excellence awards or by taking on managerial roles, all of which can boost their NHS pension pot.

One vascular surgeon who has received clinical excellence awards for outstanding medical work received a bill for more than £80,000. Dr Partha Kar, a diabetes consultant has told how the HMRC threatened to send bailiffs to his house if he did not pay a bill for an undisclosed but sizable amount.

A BMA gathering in Belfast also heard warnings that the UK would in future be unable to deploy troops abroad unless it reformed doctors’ pensions within the armed forces. Many specialist consultants had threatened to resign within the next year over the “disproportionate” impact tax rules had on them, it heard.

Dr Chaand Nagpaul, the union’s council chairman, said defence medical services, which provide healthcare to the military, would “continue to haemorrhage talented medics” if the crisis was not addressed.