Ambulance chiefs were today accused of claiming to arrive at 999 calls far quicker than they actually did.

West Yorkshire Metropolitan Ambulance Service claimed to have the best urban response times in the country by responding to more than 77 per cent of life-threatening 999 calls within an eight-minute target.

The real figure was 60 per cent in 2001/2002 - compared to the national target of 75 per cent.

A highly critical report published today by the Commission for Health Improvement (CHI) criticised the ambulance trust's figures.

The report from the independent body, set up to monitor NHS standards across England and Wales, said: "We found a substantial time lag between the time a call is received and the time the trust starts the clock to time its response.

"In some cases, the clock appeared to start after the ambulance had actually arrived at the scene."

The Trust claimed computer problems led to the discrepancy in times, but it was also found to be downgrading the life-threatening calls after they had been dealt with.

A shortage of crews, particularly at night, also had an impact on response times. Poor communication within the trust was also criticised. The report said: "The trust is experiencing teething problems with its new emergency dispatch system.

"The system does not always identify and send the nearest ambulance to an emergency. Communications between the control room and paramedics need improvement as ambulances are sometimes sent to the wrong address, or the wrong hospital is alerted to a patient's arrival."

Inspectors singled out ambulance staff for praise saying they are committed, caring and professional, but also found they are frustrated by a high workload and lack of regular breaks.

There was also praise for the patient transport service which performs well, and staff are well supported to deal with distressing incidents.

The report follows an assessment of services by CHI who paid a routine visit to WYMAS and NHS Direct West Yorkshire between September and December last year.

A trained team interviewed trust staff, local people and other organisations with an interest in the NHS, to find out how the ambulance trust is performing.

It also inspected - and praised for improving patient services - NHS Direct, a service hosted by WYMAS, which provides a confidential, nurse-led, information service to the public, via the telephone 24 hours a day.

Dr Peter Homa, CHI chief executive, said: "There are a number of areas where WYMAS needs to improve the quality of care it provides to patients and working conditions for staff.

"While leadership at the trust is innovative, the trust's progress on implementing quality improvement systems has been slow."

Jayne Barnes, director of nursing, at WYMAS, said: "There were no real surprises to us in this report, but it has provided a useful snapshot of a moment in WYMAS's development."

She said problems over response times had been caused by computers and procedures and once it had been discovered was dealt with.

An external auditor has also been called in by the trust to verify the current system for recording data is accurate.

She said the visit coincided with a move of the control centre to Wakefield and any problems highlighted were part of the "bedding in" process.

"We have made major strides forward since the review last November and we are confident that the measures we have already taken, together with the action plan developed in response to the review, will significantly improve our ability to monitor and maintain the high quality of care which local people have come to expect," she said.