The service says the build-up of traffic around the edge of the charging zone will mean drivers taking longer to get patients to hospital.

London Ambulance Service (LAS) also claims that charging will lead to more call-outs because people will not use cars to get to casualty departments in central London hospitals.

But the group behind charging - Transport for London (TfL) - is dismissing the prospect of extra tailbacks when pricing starts in February 2003.

More jams

Vehicles moving in and out of the zone will have to wait longer at red, and the LAS fears this will lead to more traffic jams.

Martin Flaherty, assistant chief ambulance officer for the LAS, told BBC London: "We believe our vehicles may experience increased delays when they've got to cross in and out of the zone and negotiate the denser traffic around the edges."

I believe that if it does work as we're planning it, then the ambulance service's fears will be unfounded

Derek Turner, Transport for London

But Derek Turner from TfL said overall traffic levels should fall and journey times should be shorter.

"I think they're concerned in case the scheme doesn't work. While I'm concerned that the scheme will work.

"I believe that if it does work as we're planning it, then the ambulance service's fears will be unfounded."

Extra journeys

It also emerged in the magazine New Civil Engineer that the LAS faces extra costs paying the charges for staff coming in to work on early and late shifts - because TfL is refusing to give them an exemption.

The payments will cost the LAS about �750,000 in the first year - the equivalent of 3,400 ambulance call-outs.

But LAS also believes an extra 28,000 ambulance trips a year may be needed in central London because of a drop in the number of people using cars to get to accident and emergency departments.

TfL has declared its willingness to talk to ambulance chiefs about their concerns.