Unison said the arrangements were not fair on crews

Because there is no 24-hour cover, paramedics say they are being called out in the early hours a few hours after finishing a regular shift.

Talks are being held with Unison over the future of stand-by cover at Bala, Tywyn, Knighton and Llanfyllin.

A union spokesman said the lives of crews, patients and other road users were being put at risk.

Newspaper reports cited an incident last month during which an ambulance crew crashed into a ditch during a call-out in mid Wales - their second call-out after finishing a shift at midnight.

It means you finish your shift, you go home, you go to bed and you can literally within minutes be called out of bed to attend to a call - maybe not once, but maybe twice or three times a shift Unison's Dave Galligan

Dave Galligan, Unison Cymru's head of health, said the overnight cover was an "unsafe, unhealthy working practice".

"The ambulance trust have, over the last couple of years, made huge inroads into reducing the number of stations that don't provide 24-hour cover - we are now left with these four stations - two in north Powys, two in south Gwynedd.

"We don't think it's fair on people who live there, or fair on the crews.

"It means you finish your shift, you go home, you go to bed and you can, literally within minutes, be called out of bed to attend to a call - maybe not once, but maybe twice or three times a shift.

"I would suggest if you have been up for 17-18 hours and maybe working for the bulk of them, you are not efficient, you are not effective and it does potentially pose risk not just for the crews, but for the patients."

He said a consultative ballot was being held - not an industrial action ballot - about willingness of crews potentially to withdraw standby cover. 'Too fatigued'

A Welsh Ambulance Service Trust spokeswoman said: "There is no legal or contractual requirement for staff on standby to work in excess of the principles set out in the working time directive.

"Shift patterns are calculated to give as much time between shifts as possible and at least an 11-hour break. If staff find themselves too fatigued to continue providing cover on standby, they should contact ambulance control and ask to be stood down.

"There is no funding currently available to upgrade these stations and recent research into call demand in the area does not indicate a need for change. The trust is looking at alternative ways of service delivery using the resources we have."

A spokesman for Health Commission Wales - in charge of funding the ambulance service via the assembly government - said the issue of providing 24-hour cover was a "matter of ongoing discussion".

He added: "The issue of cover for ambulance stations in Wales is a matter of ongoing discussion between the Welsh Ambulance Services Trust and Health Commission Wales (HCW), who will be working together on a strategy for ambulance services across Wales," he added.

The spokesman said it was looking at different ways of working which were more likely to bring about improvements to ambulance response times in rural areas.

Last month, AMs in mid Wales - Lib Dem Kirsty Williams, Conservative Glyn Davies and Plaid Cymru's Elin Jones - joined a call for urgent action over what they said were poor ambulance response times.

The cross-party group told BBC Wales that patients' lives were being put at risk.