People in Oxfordshire could be forced to wait longer for an ambulance because paramedics are taking patients to hospitals in neighbouring counties to help out Oxford's over-stretched John Radcliffe.

Under a new scheme, hospitals in Buckinghamshire, Northamptonshire and Berkshire are on standby to take emergencies from Oxfordshire when the JR casualty department is at crisis level.

But ambulances sent over the border are not available to respond to 999 calls in Oxfordshire.

Oxfordshire Ambulance Service spokesman Aubrey Bell said: "It's not a common problem, but occasionally when the JR is heaving at the rafters, we have to help out.

"Usually, when one ambulance moves, we put another in that area to replace it. But when one is forced to go out of county completely, the whole system goes down the pan."

Now Mr Bell fears the new scheme - which involves ambulances travelling to the Royal Berkshire Hospital, Reading, Stoke Mandeville Hospital, Aylesbury, Milton Keynes General Hospital, High Wycombe General Hospital, and Northampton General Hospital - is putting the service's performance at risk.

New Government targets insist paramedics arrive at life-threatening 999 calls within eight minutes.

He said: "We have a vested interest in helping the John Radcliffe if, heaven forbid, the casualty department closes. But when we take patients elsewhere, there's a potential that we won't meet our performance standards. Heads will role if we don't meet them - that has been spelt out to for us.

"Anything we can do to relieve the pressures on the JR we do, but it's a balancing act. Obviously, our ambulances are not available when they are out of the county."

In January it was revealed that the ambulance trust only managed to reach 47 per cent of emergency calls within eight minutes.

A new fleet of vehicles, including high speed Volvo ambulances, were brought in to combat the problem.

The Oxford Radcliffe Hospitals NHS Trust, responsible for the JR, has brought in the "Four County Escalation Policy" to help cut pressures in accident and emergency.

A shortage of nursing staff and cramped facilities means patients often have to endure long waits while ward beds are found for them.

In a trust board report, chief nurse Trica Hart admitted the new policy had caused problems.

She said: "Over the past few weeks a new four-county escalation policy has been used when pressures have been extreme, resulting in some referrals being re-directed to surrounding general hospitals.

"Problems currently being experienced at these times of extreme pressure are the ambulance service's difficulties in providing crews to go out of county."