Complaints about ambulance delays have risen, as new figures show that crews in Norfolk and Suffolk continue to underperform in responding to 999 calls.

The East of England Ambulance Service has warned that missing targets could scupper its bid to become a foundation trust, as it raised concerns over delays caused by ambulances queuing outside busy hospital accident and emergency departments.

The service has a target to get to 75pc of the most serious category A calls within eight minutes, which is it meeting on a regional-wide basis.

But new figures, obtained by the Eastern Daily Press under the freedom of information act, show that it failed to reach this benchmark in every single month in Norfolk in 2011, and it only reached it in three months in Suffolk.

Overperformance in counties such as Bedfordshire, Hertfordshire and Essex, means that it still reaches the regional target.

But it has failed in every month since July last year to hit the 95pc target for responding within 19 minutes for less serious emergency calls. A report to the service's board warns that if the trust fails to achieve this target for three successive quarters, it will automatically incur a red governance rating and its bid to become a foundation trust will be void.

The board papers also reveal that bad weather and 'extraordinary levels of hospital delays' at the Norfolk and Norwich University Hospital have hit its ability to meet targets in February.

In the week ending February 19, more ambulances than ever were waiting outside the region's hospitals, with nine per cent waiting more than 60 minutes.

Ambulance crews aim to hand patients to accident and emergency staff within a maximum of 15 minutes – but the average handover time increased at the three Norfolk hospitals from 2009/10 to 2010/11. The N&N was the worst performer, with its transfer time increasing from 12 minutes and 11 seconds to 18 minutes and five seconds.

N&N medical director professor Krishna Sethia said: 'We have a larger proportion of emergency admissions than other trusts in the region, cover a large rural area with an elderly population and we look after a high number of complex cases.

'We triage all patients within 15 minutes of arrival by ambulance to ensure that the sickest patients are prioritised for treatment. We are also working closely with other health partners so that more support is available to keep patients well at home and avoid emergency hospital admissions.

'Improving ambulance turnaround times requires a whole system approach and we are working together with the ambulance trust and other members of the health and social care community and actively looking for areas to improve.' A spokeswoman for the ambulance service said: 'There has been, and continues to be, a significant amount of work to reduce back up times and we expect to meet our target for the year end. Long term improvements have been made to help meet the targets more consistently going forward.

'We completed the roll out of a new computer dispatch system in the remaining two control rooms last year and while any change in a key system inevitably has a short term impact this is expected to improve performance in the longer term. More in depth assessment and appropriate treatment over the phone for people with minor injuries who don't need an ambulance means more vehicles can now be available for patients who do need them and closer working with hospitals to reduce handover delays frees up crews for other calls.

'In the highly unlikely event that we fail to meet the target the foundation trust application could be put on hold for up to six months to give us the time to prove we are consistent in achieving the targets for the future.'

The ambulance service's board meets on Wednesday, and will hear how there were 163 complaints received in the final three months of last year.

A report prepared for the board says: 'The trust saw an increase in complaints relating to delay in October, November and December – 65 (40pc of the total complaints received by the trust) of which 59 were in emergency care operations.'

In the same three months there were 16 serious incidents recorded as a result of delays.

However, there were nearly 500 compliments received from members of the public during the same period.

A service spokesman said: 'Compliments continue to far outweigh complaints received across all six counties served by the trust in the last quarter.

'The proportion of complaints is extremely low when compared to the 220,769 999 calls we received during the quarter October-December, with only 163 complaints relating to all of its services received during that time.

'We actively encourage feedback from patients to help us learn and improve services; in June 2011 a new public website went live, providing easier access for patients to feedback to the organisation.'

The trust's board will meet in Hitchin, Hertfordshire, on Wednesday at 11am.