The Government has pledged to change ambulance control room systems after two Midland services were named among six that had "misreported" their response times.
A Department of Health investigation, sparked by an anonymous call to the NHS Counter Fraud and Security Service, said West Midlands Ambulance Service and Staffordshire Ambulance Service were among half-a-dozen that had failed to record their responses to the most serious life-threatening emergency calls correctly.
The Government has set a target saying 75 per cent of such 'Category A' calls should be responded to within eight minutes.
Figures for 2005/06, prior to the reconfiguration of ambulance trusts, showed Staffordshire Ambulance Service reached 87.5 per cent of its 40,3000 Category A calls within eight minutes.
West Midlands Ambulance Service responded to 74.4 per cent of its 97,800 emergency calls within the target time.
Mis-reporting misdemeanours included starting the clock "later than the point defined by the Department of Health", clocks not being synchronised, and incorrect data management.
The four other trusts named were the former West Yorkshire and South Yorkshire (now both part of the Yorkshire Ambulance Service NHS Trust), Cumbria (part of the North West Ambulance Service NHS Trust), and the West Country Ambulance Service NHS Trusts (now within of South Western Ambulance Service NHS Trust). Geoff Catling, acting chief executive for Staffordshire Ambulance Service, said: "A little time ago following an anonymous call to the NHS Counter Fraud and Security Service, an independent investigation took place by staff from that service.
"Despite extensive work, no evidence was found deliberate falsification of response times as was alleged.
"The second search did highlight some potential areas of misunderstanding which has led to internal adjustment to the administration of the overall system. It was agreed that the very small number of inconsistencies that were highlighted did not affect the overall reporting of response figures, the veracity of which are not in question."
For years ambulance services have argued over when the clock should begin - from the second a call is received or after all vital information has been given to the operator.
But a new system, Call Connect, will be rolled out to each of the new 12 merged ambulance trusts by 2008 to ensure all response times are recorded in the same way.
Steve Parry, of West Midlands Ambulance Service, said: "Before the 31 ambulance services merged a number of different systems were being used to record response times. There was no unified approach as to when the clock should start. However, there has always been arguments over when the clock should be started, this is not an new issue in that respect.
"But now we've merged into 12 new trusts, it's much easier to implement the same system across the board, which should take effect from 2008."
Health Minister Lord Warner said the anonymous tip-off to the NHS Counter Fraud and Security Service prompted the national audit.
He said: "These irregularities affected only a minority of trusts. Patient safety was not compromised and all trusts are now reporting correctly.
"However, the Department of Health takes mis-reporting of NHS performance data extremely seriously and is determined to ensure transparency and consistency in reporting."