Ambulance bosses have responded to a report's findings that Wiltshire staff fiddled response times to 999 calls so they met government targets.

Auditors discovered more than 600 emergency response times were "altered" by the Great Western Ambulance Service between April 1 2005 and July 17 2006.

The investigation covered the period before and after April 2006, when the former Wiltshire Ambulance NHS Trust merged to become part of Great Western Ambulance NHS Trust.

The trust welcomed the report by the Audit Commission, published last week.

Tim Lynch, chief executive of the trust, said: "The report dates back to 2005/06 and we would like to stress that we have since taken a number of steps to ensure current recording is more robust and reliable.

"We're pleased that the Audit Commission has recognised that we have responded well to the recommendations and that the board is actively monitoring processes to manage data quality.

"Appropriate controls have now been out in place to ensure the accuracy of response time data, along with additional management support."

He added that the trust was implementing a new IT system that would also help it to record, monitor and predict demand levels and to manage resources more efficiently.

The ambulance-workers union, UNISON, said it was alarmed by the report's findings.

Steve Smart, branch secretary, said: "We were staggered to hear of the Audit Commission's findings.

"999 control room staff are held in high regard and respected for their integrity and hard work.

"UNISON calls for an immediate, high level investigation into these findings.

"We need an investigation to reveal if this fiddling was orchestrated by an individual, or is a reflection of institutional pressure at the non-achievement of performance targets."

He added that the union had previously raised concerns about the categorisation of emergency calls with the executive team at the trust, but were assured that there was no reason for concern.