The government today pledged to change ambulance control room systems after it emerged that one-fifth of trusts had mis-reported their response times to emergency calls.

Emergency 999 calls for ambulances reached a ten-year high of nearly 6m in England last year, with ambulance staff attending 4.8m emergency incidents.

But a Department of Health audit found that six out of 31 trusts had failed to accurately record the actual response times to the most serious life-threatening emergency calls.

The irregularities picked up in the audit included starting the clock later than the point defined by the Department of Health, and "incorrect data management".

The Department of Health today called on trusts to discipline any individuals guilty of producing faulty figures.

The ambulance service has been under pressure to meet a government target which requires 75% of such 'Category A' calls to be responded to within eight minutes.

Last year, the number of category A calls rose from just under 1m to over 1.2m, largely as a result of changes to the categories introduced last year.

Official government statistics estimate that around 74% of these calls were met within eight minutes. But the errors in ambulance trust figures made it impossible to confirm.

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.

Lord Warner, the health minister, said: "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 ambulance statistics published today show that over the past year, the ambulance service has performed well against the back-drop of a record number of calls. Crews reached over a quarter more of the most seriously ill patients within eight minutes, saving even more lives in the process."

The audit was carried out before mergers which reduced the number of ambulance trusts from 31 to 12.

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.

Healthcare unions blamed a "systems failure" for inaccurate response time logs. Mike Jackson, Unison's senior national officer, said: "The fact that there have been some mistakes over recording of response data in a minority of trusts is mainly down to systems failure and confusion over when 'the clock starts ticking'.

"Clarity is important and these mistakes appear to have been rectified but we want to ensure that staff are given clear recording instructions, so that accurate response times are recorded across all ambulance trusts."