The ambulance service in the South West is at the centre of a major political row after it emerged the region has the worst response times in the country.
Latest figures show only 68% of ambulances reached the scene of the most serious category A cases within eight minutes in October.
This was the lowest out of all 11 ambulance trusts and compares to 74.6% nationally for the most time-critical calls.
The data was released following a Freedom of Information request by Labour - which showed waiting times have been increasing for the last two years.
The South Western Ambulance Service was one of four trusts which failed to hit the required 75% target for the most urgent critical calls.
It was also one of six which missed the target for responding to serious but less time-critical calls on time.
But the news drew a stinging response from South Western Ambulance Service chief executive Ken Wenman, attacking shadow health secretary Andy Burnham.
He said the information "has been misinterpreted and misreported in order to present a grossly inaccurate picture for the purposes of apparent political gain".
But Mr Burnham said 999 response times are getting worse.
He said: "The Government's failure to face up to their A&E crisis is having a serious knock-on effect on ambulance services
"More and more calls are being attended by police cars and taxis on David Cameron's watch. What clearer sign could there be of an NHS not safe in Tory hands? Patients deserve better."
Last week Labour pointed out that South West ambulance bosses spent £102,483 on taxis last year and that the average response time has increased nationally by 30 seconds because of queues at A&E departments.
However, the Conservatives said Labour were scaremongering.
A spokesman said: "We know ambulance services are under pressure, with more people needing more healthcare as our population ages.
"But Labour's scaremongering is disingenuous; thousands more people are being seen within the ambulance target time since the election."
Mr Wenman said taxis are never used in life threatening situations.
He wrote: "Taxis are only used to transport patients in a very small minority of cases where it is clinically safe and appropriate to do so.
"Taxis would never be used to transport patients in emergency or life-threatening situations."
A trust spokesman said: "Like all ambulance trusts around the country, the service is facing an ongoing increase in demand for its services meaning more calls to respond to and increasing numbers of patients requiring care.
"An additional challenge for the south west region is that it is predominantly rural, with many isolated communities. This area of the country also has the highest percentage of elderly people who are more likely to access our services, especially in the run up to and during winter.
"The trust would like to make an assurance that the provision of high quality emergency and urgent care services remains our top priority."
