Ambulances are being used as waiting rooms outside hospital emergency units in order to meet Government pledges on treatment times, a union has claimed.
One report said patients were left in ambulances for five hours
Unison said so-called patient stacking in ambulances was used if they could not be seen in A&E within four hours.
The Department of Health denied the target was causing undue delays.
It said the four-hour limit for A&E waiting starts 15 minutes after the ambulance arrives, regardless of whether a patient has been handed over.
But Tories and Liberal Democrats accused the government of mishandling the NHS.
Shadow health minister Mike Penning said: "Not admitting people to hospital but stacking patients in car parks beggars belief in the 21st century."
Lib Dem health spokesman Norman Lamb said it was evidence of a "shocking systemic failure" in emergency care services.
The claims come after the Observer reported that some patients were left for up to five hours in an ambulance because A&E refused to admit them until they could seen within the target time.
A study of seven of England's 11 regional ambulance services by the newspaper showed delays of more than an hour were common and that at least 44,000 delays were reported in the last 15 months.
Unison - which represents ambulance staff - said it first raised the issue in 2005.
Spokeswoman Mary Maguire said: "A 16-year-old terminally ill cancer patient died after waiting over an hour for an ambulance to transfer him. Three ambulances could have reached him but they were tied up waiting to hand over patients to A&E."
"It happens time and time again. It is an appalling waste of resources. We should not use ambulances as waiting rooms."
A Department of Health spokesman said: "The turnaround time does not reflect just the time spent by patients in the ambulance before being admitted to accident and emergency time but also time spent cleaning, restocking and readying the ambulance to go back out on the road."
He added: "Four years ago, almost a quarter of patients spent more than four hours in A&E - now, over 98% are seen, diagnosed and treated within four hours of their arrival."