The Great Western Ambulance Service is overspent by £690,000 since April.
Part of the overspending, which is across the service as a whole, is due to paying private companies and overtime due to sickness, maternity leave and training.
Despite this the service, which covers Wiltshire, Gloucestershire and Avon, did not achieve the national target of answering Category A calls (life threatening) within eight minutes 75 per cent of the time in July. It achieved 72.5 per cent.
The board of GWAS was warned that the service does not currently have the capacity to meet the targets.
Simon Truelove, interim deputy director of finance at GWAS, told a board meeting last Thursday: "We have to be doing 27,000 hours a week of accident and emergency ambulance activity but the budget assumes 24-25,000 hours which is a gap of £700,000 a month."
He said that GWAS hopes to get the funding from Primary Care Trusts in the three areas but said this would only be forthcoming if targets were met.
The overspend of £690,000 includes £286,000 in the headquarters of the trust mainly due to an over establishment of staff in training and personnel, some of which is covered by agency staff.
Tim Lynch, chief executive of GWAS, said the staffing situation would improve in September.
He said: "Fifty nine emergency care practitioners will complete their university course and be available for roster.
"We are investing in our staff. No one comparable to our size is doing as much as we are doing with training."
Ian Whittern, chairman of the GWAS branch of the union Unison, said: "The overspending can be attributed to the target chase for response times.
"They are employing 11 different private contractors to try and increase cover to demonstrate they are achieving some of these times."
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