Staff at a third of ambulance trusts are altering data on response times to meet government targets, a report says.
There are tough targets for response times
Ambulances are supposed to arrive at the most urgent calls within eight minutes, at least 75% of the time.
A report by the Commission of Health Improvement found a 20% improvement in response times since 1999.
But it said genuine improvements had been accompanied by some fiddling of the figures.
The report praised frontline staff for their dedication, but said some had come under intense pressure to drive down response times, and it criticised the intimidating management culture in some trusts.
Staff did not feel able to raise concerns without fear of suffering adverse consequences, it said.
Jocelyn Cornwell, CHI's acting chief executive, stressed that there had been significant improvements in performance.
But she said: "While we feel that targets have been helpful in focusing the
minds of ambulance trust staff on improving performance, we believe that more sophisticated measures of response times and outcomes measures are now needed."
CHI also noted that the strong focus on meeting the category A target meant
there was less focus on other areas, such as training and other targets.
The Department of Health said hundreds of lives were being saved due to three-quarters of trusts reaching the 75% urgent response target.
But a spokesman said: "We recognise that response times can be recorded inconsistently and sometimes inaccurately and these issues are already being addressed."
Richard Diment, chief executive of the Ambulance Service Association, acknowledged there had been some evidence response times were being manipulated.
But he said: "The ASA notes that the watchdog says its findings do not point to a widespread 'fiddling of figures'."
Struggling to cope
Shadow health secretary Dr Liam Fox said government targets had done nothing to improve the service.
"The ambulance service is clearly struggling to deal with the genuine needs of patients because of the government's emphasis on statistical targets," he said.
"The image of ambulances queuing outside accident and emergency departments, unable to answer patient calls because hospitals won't accept patients for fear of breaking their four-hour A&E targets, confirms what the Conservative Party has long been telling the government.
"This is yet another sad example of statistics taking precedent over patients in Labour's target-obsessed NHS."
The Consumers' Association (CA) criticised CHI's report, saying it was short on detail and failed to tackle many crucial issues.