Patient handovers at hospitals 'delaying ambulances'
Ambulances spent more than 80,000 hours waiting to transfer patients to hospitals in two years, claim the Welsh Liberal Democrats.
Turn-around times when handing patients to hospital staff should take no longer than 20 minutes, under targets.
The Lib Dems claimed the figures they obtained showed thousands of hours of emergency ambulance cover were lost.
The Welsh Assembly Government said most patients were treated and transferred in an appropriate time.
Between January 2009 and October 2010, turn-around times beyond the 20-minute allowance amounted to more than 80,400 hours - or 3,350 days.
Waits totalled 15,909 hours at the Royal Gwent Hospital in Newport. while Morriston Hospital in Swansea was second with 11,926 hours, ahead of Cardiff's University Hospital of Wales with 9,986 hours.
The figures from the Welsh Ambulance Service NHS Trust also include three hospitals in England.
The service calculates that the cost of a "lost unit hour" is £76, meaning the long turn-around times cost an estimated £6.1m.
Welsh Lib Dem health spokeswoman Veronica German said the figures, obtained through a freedom of information request, were "absolutely shocking".
"This proves that there is systemic failure in the way the Welsh NHS handles emergency situations," she said.
"Month after month, we see that Welsh patients have to wait an unacceptable amount of time for ambulances to respond to emergency call-outs and now we see that ambulances have to wait an unacceptable amount of time to transfer patients and get back on the road to respond to emergency calls."
Ambulance staff were working hard in difficult circumstances to improve the service, Mrs German said.
"But it is clear that ambulances cannot attend calls if they are queuing outside hospitals," she added.
An assembly government spokesman said emergency departments were under "constant pressure".
"We must remember that the ambulance service receives more than 25,000 emergency calls every month, the majority of which result in an ambulance taking the patient to an emergency department," he said.
"Unfortunately there will be occasions when there is delay in handing over the patient to hospital staff."
He added: "A significant amount of work has been done to reduce the level of delays and we are working with health boards and the Welsh Ambulance Service Trust to secure the further progress required.
"All patients are dealt with in order of their clinical priority, with the majority of patients being seen, treated, transferred or discharged in an appropriate time."
An ambulance service spokesman said: "We are working hard with hospitals to reduce lost hours outside A&E departments.
"We are developing alternative care pathways to help manage patient demand by getting the right care to the right patient, which may not mean A&E treatment.
"However, the ambulance service and hospitals face high demand during the winter and summer holidays through flu and norovirus outbreaks and increased tourist population which can result in delays."
He said people could help the NHS by only using A&E or dialling 999 when they were badly injured or showing symptoms of critical illness.