NHS ambulance services across England are now spending close to double the figure on private ambulances when compared to 2012, with parts of the country seeing a ten-fold increase.
Labour Freedom of Information requests to all ambulance trusts reveals an increase of 82% between financial years 2011/12 and 2013/14 - from £37 million to £67.5 million last year.
Data released earlier this month by NHS England shows ambulances in some regions taking an average of almost four minutes longer to reach patients in a life-threatening condition. Parts of the country where private ambulance spending is highest are among the slowest responders to calls.
In the North East region, spending on private ambulance services more than quadrupled to hit £2.9 million.
Yet other ambulance services maintained lowest levels of spending across the period while one even reduced its reliance on private vehicles.
Over the same period, average ambulance response times - the period between a logged call and the vehicle's arrival - increased by 51 seconds in the North East area.
Official NHS figures show that across the country even ambulances for the most serious cases are taking over a minute longer to reach patients than three years ago.
In cases where a life is threatened, the vehicle should arrive within eight minutes. The average ambulance in England now takes 6 minutes 36 seconds to reach callers - 78 seconds longer than in August 2011.
Andy Burnham, Labour’s Shadow Home Secretary said:
"These figures show just how quickly the NHS is changing under David Cameron. Blue-light ambulance services have traditionally been considered part of the public core of the NHS. It is clear that no part of the NHS is now immune to privatisation.
"When people dial 999, most would expect an NHS ambulance crew to turn up. People have never been asked whether they think blue-light ambulance services should be run by private companies. Before this practice goes any further, there should be a proper public debate about it.
"NHS paramedics have raised concerns over whether private crews have sufficient training, competence and are fully equipped. The Government needs to provide urgent answers to these questions and provide assurances that this practice is not compromising patient safety.
“Cameron's Government is driving the private sector into the core of the NHS without the permission of the public. It is wrong and should be stopped. If the Prime Minister wants to continue to subject the NHS to increasing privatisation, he must seek a specific mandate for it at the coming Election."