More than half the ambulance services in England and Wales have been providing inaccurate performance data, the BBC has learnt.
Questions have been raised by many trusts
The Commission for Health Improvement has discovered widespread discrepancies in the way ambulance response times are recorded.
The discovery follows a report into West Yorkshire Metropolitan Ambulance Service.
It found officials there have been exaggerating the speed with which they answer the most urgent 999 calls.
Ambulance services have to record how quickly they respond to the most urgent 999 calls.
In at least three quarters of these incidents, an ambulance should arrive at the scene within eight minutes.
West Yorkshire Ambulance Service claimed it was beating that target. It said it achieved the eight minute response time in more than 77% of the most urgent calls.
But when inspectors from CHI investigated they found the speed with which the ambulance crews responded was being exaggerated.
They believe the true figure for the trust's response time was closer to 60%, well below the government's target.
A spokesman for the commission said: "What was happening in this instance was that they weren't beginning the clock at the point where they should have been so they were taking more time talking to the person on the phone while the ambulance was already running to the incident which made their journey times look shorter than they were."
Managers at the trust have denied claims they were fiddling figures.
A spokeswoman said: "We do take over 1,000 calls per day for help and this issue was a pure systems issue and we are confident the system is now operating effectively."
CHI is now investigating other ambulance trusts to see if their figures are inaccurate. The results are due to be published later in the year.
However, a spokesman told the BBC there is evidence of discrepancies in 15 out of 28 trusts surveyed so far.
While some of these are minor, it is understood that others have caused concern.
The Department of Health said it is looking at how best to 'tighten up' procedures and resolve this issue once and for all.
Shadow Health Secretary Dr Liam Fox MP said: "These shocking findings show more than half of the trusts so far investigated throughout the country to be systematically fiddling the figures.
"This is not only scandalous, it also lures people into a dangerous false sense of security.
"It is a shocking indictment of the government's mismanagement of the health service that medical and managerial staff are under such pressure to fiddle figures.