The trust said it had to prioritise but did "not always get it right"
South Central Ambulance Service has admitted it is not responding quickly enough to 999 calls from rural areas.
Hampshire, Berkshire, Oxfordshire and Buckinghamshire councillors accused it of operating a "two-tier" system with response times higher in urban areas.
Several cases of ambulances arriving late or failing to turn up have been reported in 2009.
South Central Ambulance Service NHS Trust said a rising number of calls had put it under "huge pressure".
Will Hancock, chief executive, admitted that although the service was meeting the eight-minute target for urban areas, it takes on average 11 and half minutes for ambulances to respond to emergency calls in rural areas.
The service provides 999 cover for a population of about four million people and deals with more than 400,000 emergency and urgent calls every year.
A joint review group consisting of Health Scrutiny Committee members from the four county councils met last week and called for urgent improvements to response times.
Its chairman, Dr Peter Skolar, said: "They have to have 75% [of life-threatening emergencies] responded to within eight minutes.
"If you look at their figure they comply with that target. Broken down into city, town and urban, as against rural, and then you see that they fail dramatically."
Mr Hancock said: "There is a limited supply of ambulances and we have to prioritise quite carefully and we don't always get that right.
"We welcome the inquiry. It's highlighted what is actually a national issue and a challenge to all ambulance services.
Chief executive Will Hancock admitted the trust was not meeting targets in rural areas
"We understand the concerns that rural communities have and we do focus on improving responses to patients in rural areas.
"However we do put the majority of our ambulances where the majority of out patients live, and it is a fact that we are far busier in urban, rather than rural areas."
Several patients have criticised the ambulance service's response times.
The most recent was Kevin Fenlon, a fuel pump technician who was working at a garage in Chandlers Ford, Hampshire, when he was hit by a van on 18 November.
He suffered a badly broken ankle and phoned for an ambulance but said he was told he "didn't need" one.
Mr Fenlon's manager picked him up and drove him to hospital where he underwent surgery for four hours.
He said medical staff told him he could have lost his foot because of the delay.
The ambulance service has since apologised.
The Health Scrutiny Committees' report is due to be released before Christmas.
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