Half of England's ambulance trusts are missing response targets despite a service shake-up designed to improve performance.

Last July, 31 ambulance trusts were merged into 12 in order to improve performance, but latest Ambulance Service Association figures reveal that six of the 12 trusts are failing to answer emergency category-A calls within the benchmark eight minutes.

At the end of the last financial year, almost three-quarters of the 31 ambulance trusts hit the 75 per cent target, but the ASA says the restructuring, an increase in demand and inadequate primary care trust commissioning have all contributed to the fall in performance.

ASA chief executive Richard Diment said: 'People have needed time to adjust to reconfigurations, not only in ambulance trusts, but among the commissioning bodies.'

He criticised primary care trusts for prioritising finances and the 18-week maximum waiting target over ambulance performance.

'There are a number of trusts that are not being commissioned to sufficient levels - this is an area of extreme frustration,' he said.

The target was originally due to be toughened when the ambulance trusts were reconfigured last July, with the intention that the clock would start from the moment a call was made rather than when all a caller's details had been taken.

But this was delayed by a year until 2008, following pressure from ambulance trusts, which said it would have made the 75 per cent target impossible to hit.

North West, Great Western, South East Coast and London told HSJ they are among the trusts currently missing the category-A target. Some say they hope to catch up by the end of the financial year.

Other trusts have recorded huge variations in the different patches they cover, suggesting that high-performing areas could be masking low achievement elsewhere.

For example, in South Western Ambulance Service, Dorset consistently hits the target, while Somerset and Cornwall have failed every month since last July.

Trust chief executive Ken Wenman said: 'It is historically more difficult to achieve these exacting performance targets in more rural areas. However, this has been recognised by our commissioners, and work is being undertaken to assess the financial implications of meeting them.'