The report, by the head of Staffordshire Ambulance Service, suggests many trusts manipulate figures to make it look like they are meeting government targets.

It suggests some delay starting clocks by as much as three minutes to ensure they are seen to be responding to emergency calls within the eight-minute target.

These are very worrying allegations

Department of Health spokesman

The findings back up previous claims by patients groups and have been described as "worrying" by the Department of Health.

Ambulances are supposed to respond to 75% of all high priority calls within eight minutes.

Unreliable figures

Figures published by the Department of Health in July suggested that 29 out of the 32 ambulance trusts in England met this target.

However, the report by Roger Thayne, chief executive of Staffordshire ambulance service raises serious questions about the reliability of those figures.

His report found that one ambulance trust managed to reach 3.5% of its life-threatening calls in less than "zero minutes".

Three trusts claim to reach one in 10 life threatening calls within two minutes and 17 say they reach one in 10 high priority patients within three minutes.

Mr Thayne's report - details of which have been leaked to national newspapers and the Consumers' Association - said such results were practically impossible.

He said that it takes an average of two minutes for an ambulance to be dispatched after a call has been made.

These revelations make a nonsense of the government's claims about ambulance response times

Dr Liam Fox, Shadow Health Secretary

Mr Thayne declined to comment on his report on Thursday. A spokesman for Staffordshire ambulance service told BBC News Online: "It was a confidential report and as a result there will be no comment from us."

But Dr Anton van Dellen, medical adviser to the service, said: "The message from this report is that there is a concern as to the reliability of some of the response data."

Fiddling times

The Consumers Association said the report backed up its claims that response times were being fiddled.

Sheila McKechnie, its director, said: "This report is a complete vindication of the research published by Health Which? last month.

"It demolishes the claims made by the Department of Health and the Ambulance Service Association that response time figures are reliable and that the government's targets are being met."

She called on the Department of Health to launch a full investigation into response time figures.

A spokesman for the Department of Health said: "No evidence of widespread misreporting has been reported to us.

"However, these are very worrying allegations and we will look closely at the ASA's final recommendations."

The Ambulance Service Association is currently working on a report into variations in response times.

Its chief executive Richard Diment said: "We are trying to work out what those variations are and put some recommendations to the Department of Health so we can have some consistency."

Speaking to BBC News Online, he added: "There might be some variation but we are probably talking 15 seconds in eight minutes rather than the three minutes suggested by Mr Thayne."

A spokesman for the Association of Professional Ambulance Personnel added: "The only way forward to address the contentious issue of ambulance response times is to ensure all ambulance trusts record the arrival times of thir crews in the same way. Only then, will this controversial issue be adequately addessed."

Shadow Health Secretary Dr Liam Fox said: "These revelations make a nonsense of the government's claims about ambulance response times."

He added: "Do ministers really believe that any of their boasts are credible when it is claimed that an ambulance would reach a victim before it has even been called?"

Liberal Democrat health spokesman Dr Evan Harris said: "This is yet another example of the way government-imposed targets lead to treatment of figures rather than treatment of patients."