Hundreds of patients were left waiting up to an hour in ambulances outside Lancashire Teaching Hospitals this winter, contravening a key NHS safety standard.
Unison said delays risk the safety of patients and are "a terrible waste" of ambulance crews' skills.
Ambulances took 6,323 patients to emergency departments at the Lancashire Teaching Hospitals NHS Trust between December 3 and February 3, according to NHS Digital data.
Of those, 284 waited between 30 and 60 minutes before A&E staff were able to take over from paramedics, and 58 arrivals waited for more than an hour.
Despite the NHS saying a delay of just 15 minutes is a potential threat to life, 23 patients waited up to an hour in a single day when the problem was at its most severe.
The overall number of delays at Lancashire Teaching Hospitals has plummeted since 2017-18.
Over the same period last winter, 2,366 patients waited in ambulances for 30 minutes or more, 86 per cent more than this year.
Colm Porter, Unison's national ambulance officer, said: "Handover delays are a stark illustration of the extreme pressure on the NHS caused by years of chronic underfunding.
"Not only are patients put at risk with lengthy waits to get into A&E, but there are also fewer ambulances available to respond to emergencies.
"Ambulance staff are stuck in hospitals when they should be out responding to life-or-death 999 calls. This is a terrible waste of their skills and the stress it causes can soon takes its toll on their health."
The Society for Acute Medicine said emergency staff are only too aware that waiting in the back of an ambulance is inherently less safe for patients.
President Dr Nick Scriven said: "Staff will be forced to make horrible decisions about who gets the next space when one becomes free, fully aware that any mistakes could be catastrophic."
NHS bosses have stressed that A&Es should always accept handover of patients within 15 minutes of arrival by ambulance.
An NHS spokesperson said: "Thanks to the hard work of ambulance and hospitals more people have benefited from a timely handover this year, despite a significant increase in people requiring care.
"We remain committed to further reducing delayed handovers, so that ambulance crews are able to get back out on the road and providing care to patients sooner.”
Across England, nearly 100,000 people waited for 30 minutes or more between December and February - one in 10 arrivals.