Thousands of ambulances are being prevented from responding to life-threatening 999 calls every day because they are tied up at A&E units, in a significant continuing breach of NHS patient safety rules.

The Guardian can reveal that almost 600,000 ambulances faced delays of more than 15 minutes in their crews handing a patient over to hospital A&E staff in the past three months – something NHS rules say should never happen.

Previously unpublished NHS figures show for the first time that the problem of A&E handover delays is three times worse than that portrayed in NHS England’s official statistics.

Q&A Why is the NHS winter crisis so bad in 2017-18? Show A combination of factors are at play. Hospitals have fewer beds than last year, so they are less able to deal with the recent, ongoing surge in illness. Last week, for example, the bed occupancy rate at 17 of England’s 153 acute hospital trusts was 98% or more, with the fullest – Walsall healthcare trust – 99.9% occupied. NHS England admits that the service “has been under sustained pressure [recently because of] high levels of respiratory illness, bed occupancy levels giving limited capacity to deal with demand surges, early indications of increasing flu prevalence and some reports suggesting a rise in the severity of illness among patients arriving at A&Es”. Many NHS bosses and senior doctors say that the pressure the NHS is under now is the heaviest it has ever been. “We are seeing conditions that people have not experienced in their working lives,” says Dr Taj Hassan, the president of the Royal College of Emergency Medicine. The unprecedented nature of the measures that NHS bosses have told hospitals to take – including cancelling tens of thousands of operations and outpatient appointments until at least the end of January – underlines the seriousness of the situation facing NHS services, including ambulance crews and GP surgeries. Read a full Q&A on the NHS winter crisis

The latter only record delays of 30 minutes or more, even though any wait of more than 15 minutes is deemed a potential threat to life by the NHS, because it means ambulances are not available to respond to the most serious medical emergencies, such as those involving victims of strokes, stabbings, heart attacks or breathing problems.

Those statistics show ambulance crews in England had to look after 186,000 patients, either in the back of their vehicle or in a hospital corridor, for at least half an hour between late November and early March.

However, the data detailing handover waits of at least 15 minutes – obtained from England’s 10 regional NHS ambulance services – show incidents that the health regulators NHS England and NHS Improvement say should not happen occurred 594,279 times during December, January and February.

Senior figures in charge of acute, urgent and emergency care at the regulators wrote to all NHS trusts on 15 November, spelling out the dangers of delayed handovers. In a letter obtained by the Guardian, they stressed: “Acute trusts must always accept handover of patients within 15 minutes of an ambulance arriving at the emergency department (ED). Leaving patients waiting in ambulances or in a corridor supervised by ambulance personnel is inappropriate.

“Delays in handover of patients from ambulance services to EDs result in: increased risk to patients on site due to delays in diagnosis and treatment [and] increased risk in the community because fewer ambulances are available to respond.”

Highlighting the risk to patients waiting for an ambulance to respond to a 999 call, they said: “The patients in the urgent care pathway who are at the highest risk of preventable harm are those for whom a high-priority 999 emergency call has been received, but no ambulance resource is available for dispatch.”

The disclosure that hospitals are routinely flouting a key NHS safety standard has prompted warnings that understaffed A&Es are often unable to cope and patients could die as a result.

Ambulance service chiefs warned that lives were at risk because delays were so common. The Association of Ambulance Chief Executives said: “We are concerned that the continued increase in the number of transfer-of-care delays at many NHS hospitals has reached critical levels and is now directly affecting the quality of care received by patients.”

A spokesman added: “The problem is most acute when ambulance trusts are left with no resources available to respond to the most serious, life-threatening 999 calls because they are stuck at hospital waiting to transfer patients into the care of the hospital.”

Dr Nick Scriven, the president of the Society for Acute Medicine, which represents hospital doctors specialising in emergency medicine, said: “This is clearly showing a massive risk not only for those waiting to get into hospital, but also those waiting for an ambulance to attend to them in the community after a call for help. This is another breach of a target put in place to maximise patient safety.”

At some hospitals, up to 88% of ambulance arrivals during those three months took longer than 15 minutes to hand over because their emergency departments were so heavily overcrowded and staff were busy looking after unprecedented numbers of people.

For example, 87.9% of handovers at Watford general hospital took more than 15 minutes. At Queen Elizabeth hospital in King’s Lynn, Norfolk, 87.4% of handovers in February took that long, as did 85% of all handovers during winter at Stepping Hill hospital in Stockport. Similarly, 2,485 (72%) of the 3,443 handovers at Royal Blackburn hospital in December flouted NHS rules.

Of those, 1,150 took 15 to 30 minutes – breaches that did not show up in the figures released by NHS England, because only data for waits of at least 30 minutes was published.

A busy A&E department at Queen’s Medical Centre in Nottingham. Photograph: David Sillitoe/The Guardian

South East Coast ambulance service figures show Royal Surrey County hospital in Guildford – the main hospital providing care to voters in the South West Surrey constituency of the health secretary, Jeremy Hunt – had the highest percentage of 15 to 30-minute handovers during the three months of any hospital in its area. More than half (54.1%) took that length of time.

Kevin Brandstatter, the NHS national officer at the GMB union, which represents 15,000 ambulance staff in England, said: “We are very concerned at the threat to patient safety. The inability of trusts to simply admit patients to hospital threatens lives.”

Understaffed A&Es in hospitals with too few beds cannot cope with demand for care, he added.

“The shortage of clinical and reception staff in A&E departments mean that when ambulances arrive, there is no one to hand over to. That leads to longer waiting times for 999 callers, including elderly people not being attended to after falls in the street, ambulance response times being missed, and inevitably the deaths of patients who cannot be reached,” Brandstatter said.

Saffron Cordery, the deputy chief executive of NHS Providers, which represents NHS trusts, said: “These figures highlight the scale of the task faced not just by ambulance trusts, but all health and social care services, in ensuring safe and timely care for patients in the face of a steep and relentless rise in demand with limited resources.”

NHS Improvement said: “While patients in ambulances still receive care from qualified medical staff, it is clearly far from ideal. Getting patients out of ambulances and into emergency departments quickly is best for them and means the ambulances can get back on the road to help others.

“We’re working hard with trusts to help them deal with the huge demand [from] people who are using A&E departments, particularly over winter.”

