Thousands of ambulances are being delayed by queues at A&E every day, preventing them from responding to emergency calls, according to a report.
Figures obtained by The Guardian suggest almost 600,000 ambulances faced delays of more than a quarter of an hour in the last three months - in a breach of NHS patient safety rules.
The findings are three times worse than figures portrayed in NHS England's official statistics, the paper reported.
In December, January and February, figures obtained by The Guardian show that at England's 10 regional NHS ambulance services, 594,279 arrivals took at least 15 minutes to hand over their patients.
At some hospitals, nearly 88% of ambulance arrivals took longer than 15 minutes to hand over because the A&E units were so full.
In a letter sent to all NHS Trust chief executives on November 15 2017, senior NHS figures said: "Acute trusts must always accept handover of patients within 15 minutes of an ambulance arriving at the Emergency Department (ED) or other urgent admission facility.
"Leaving patients waiting in ambulances or in a corridor supervised by ambulance personnel is inappropriate.
"The patient is the responsibility of the ED from the moment that the ambulance arrives outside the ED department, regardless of the exact location of the patient."
According to the Guardian, 87.9% of handovers at Watford general hospital took longer than 15 minutes, while 87.4% of handovers at the Queen Elizabeth hospital in King's Lynn took as long.
It added that in December at the Royal Blackburn hospital, 2,485 of 3,443 handovers took at least 15 minutes, representing 72%.
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."