There are wide variations in the way ambulance trusts in England calculate their response times to 999 calls, evidence seen by the BBC suggests.
Most trusts in England claim to have hit a response time target
Many trusts do not start timing responses until minutes after receiving a call, it has emerged.
Latest figures show that most ambulance trusts in England claim to have hit a government target on response time.
But they do not show that trusts count response time from different starting points, the BBC's Andrew Hosken said.
In 2001, the government said ambulances should reach at least 75% of life-threatening emergency calls in eight minutes - the maximum time a cardiac arrest patient has to be seen if they are to have any chance of surviving.
According to the latest figures, 26 out of the 31 ambulance trusts in England claim to have hit the target or bettered it.
But it has emerged that many trusts do not start the clock until a minute-and-a-half to two minutes after receiving a call, the BBC investigation found.
There are also wide discrepancies among ambulance trusts as to what they term as a life-threatening emergency.
Roger Thame, chief executive of the Staffordshire Ambulance Service, the country's best-performing ambulance trust, said the late clock sometimes enabled ambulance services to achieve targets and could be masking poor performance.
"We are cheating the system. We need to be open and honest about how long it takes.
"I think it takes one-and-a-half to two minutes (before starting the clock) on average, and in some cases over four minutes."
The BBC investigation showed some ambulance trusts were achieving extraordinary results by reaching calls within one second.
And an NHS source said the London Ambulance Service, which claimed to have met the government target last year, had received a report which showed the London control room delayed starting the clock by an average of four-and-a-half minutes.
A spokesman for the London Ambulance Service said he could not discuss the report because it was prepared for the government.
But he said over the last six months, the clock started on average 83 seconds after receiving a call.
The investigation also raised concerns about how trusts are categorising life-threatening calls.
Figures from 2005 showed that only 8% of calls in Berkshire were in the most serious group - compared to a national average of 30% and 46% in Staffordshire. The trust said it stood by its figures.
Richard Diment, chief executive of the Ambulance Service Association, said government guidelines demanded trusts only start the clock after certain information had been collected from the caller.
He said a few years ago some trusts may have been taking four minutes, but he added: "I think most trusts are starting the clock within one minute."
Health Minister Lord Warner also denied the situation was as bad as the BBC investigation suggested.
"In any human system there will be bits of frailty, but all ambulance services have improved in the last few years and that has saved lives."