The Government pledged to change ambulance control room systems today after six trusts were found to have "mis-reported" their response times.

A Department of Health audit found that six trusts had failed to record correct details for how they responded to the most serious life-threatening emergency calls.

The Government has set a target saying 75 per cent of such 'Category A' calls should be responded to within eight minutes.

The trusts are the former West Yorkshire and South Yorkshire (now both part of the Yorkshire Ambulance Service NHS Trust), Cumbria (now part of the North West Ambulance Service NHS Trust), West Midlands (now part of the West Midlands Ambulance Service NHS Trust), the West Country Ambulance Service NHS Trusts (now part of the South Western Ambulance Service NHS Trust) and the unchanged Staffordshire Ambulance Service NHS Trust.

All trusts had now corrected their procedures but follow-up audits "will take place to check that they are now reporting correctly", the Department of Health said.

The mis-reporting included "starting the clock later than the point defined by the Department of Health, incorrect data management and the clocks on different servers not being synchronised which meant that time taken to respond was not calculated correctly," it added.

The news comes as official figures showed that ambulance 999 calls have reached a 10-year high.

Health Minister Lord Warner 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.

"That is why we accepted, and have begun to implement, the recommendations in last year's ambulance review - Taking Healthcare to the Patient - around necessary changes to performance reporting.

"As soon as issues around certain ambulance trusts' data came to our attention we undertook a full audit of all trusts.

"In the areas where there have been instances of reporting irregularities, we have asked those trusts to consider whether there is an organisational or individual case to answer, and if so to take appropriate steps, including any necessary disciplinary action.

"We will be undertaking follow-up audits to check that all trusts are continuing to report data in line with our guidance."

The Department of Health said any relevant changes to computer systems that record response times now need to be pre-approved by itself to ensure consistency.

It added: "The Department has also reached agreement with software suppliers to change control room systems so that trusts cannot make changes to their own systems - the supplier would need to make the change, following approval by the Department.

"These new arrangements will come into effect from April 1, 2007."

Figures released today from the Information Centre for Health and Social Care showed that six million emergency ambulance calls were received in England between April 2005 and March this year.

That was 6 per cent up on the previous year's figure (5.6 million) and almost double the 3.2 million calls made in 1995/96.

As a result, ambulance services attended 4.8 million emergency incidents last year, the figures showed.

Despite the mis-reporting by the six trusts, the national figures showed that a record number of Category A calls were responded to within eight minutes - up more than a quarter on the year before.

In 2004/05, just under a million of the most serious emergency calls were responded to within eight minutes but that rose to more than 1.2 million in 2005/06.

An estimated 74 per cent of those calls received a response within eight minutes - just short of the Government's 75 per cent target.

In total, 96 per cent received a full response within 14 minutes in urban areas, and within 19 minutes in rural areas.

Across all types of calls, patients were taken to hospital on 73 out of every 100 occasions.

And ambulance crews made 788,000 journeys transporting urgent cases to hospital, as requested by doctors, down 7 per cent on the 849,000 made in 2004/05.

The centre's chief executive, Professor Denise Lievesley, said: "It is encouraging to see that ambulances responded to nearly a quarter of a million more immediately life-threatening calls within eight minutes last year."

Regarding the mis-reporting, she added: "The Information Centre for Health and Social Care takes the issue of data quality very seriously and, together with the Department of Health, we will identify and implement any appropriate requirements for data quality assurance."