East Midlands Ambulance Service (EMAS) spent £900,000 more on fuel than it originally planned in the past financial year, bosses have said.
Brian Brewster, director of finance at EMAS, said rising fuel costs were a real challenge for the service.
For the year 2010/11, EMAS spent £6m on fuel, which works out as £500,000 a month.
Mr Brewster said there was some flexibility in its budget and front line services would be protected.
"We have to live within the resources we have," said Mr Brewster.
"We spend £161m a year and fuel represents 3.8% of that total spend.
"We will endeavour to maintain our services at all times. We will prioritise our spend and protect front line services at all costs."
Andrew Howard, from motoring organisation the AA, said times were hard for all services.
"This is something which affects everyone's budgets and is driving up the cost of everything because everything in this country is shifted around by lorry," he said.
"All organisations have to have some flexibility in their budgets and have to take rising fuel costs into consideration."
Mr Brewster said EMAS had budgeted an extra 10% for fuel for the coming year.