Labour has warned of a risk to patient safety after figures revealed a significant rise in the use of private sector ambulances by some trusts.
Shadow health secretary Andy Burnham said people would be stunned that "blue-light 999 services" were being privatised without proper debate. He will write to Health Secretary Jeremy Hunt to ask for an urgent assurance about the safety and quality of all privately-provided emergency ambulance services.
Freedom of Information requests submitted by Labour revealed increases on spending by three English ambulance services on private contractors in the last two years. South East Coast Ambulance service spent £7.3 million in 2012/13, up from £1.9 million in 2010/11. In Yorkshire the spend on private contractors went up from £500,000 to £1.8 million in the same period and in London it rose from under £400,000 to £4.2 million.
Mr Burnham said: "When people dial 999, they don't expect a private ambulance to turn up. This news will open people's eyes to the way the NHS is changing under David Cameron. People will be stunned to learn that even blue-light 999 services are being privatised without proper debate. It is proof that the Coalition sees no limits on privatisation in the NHS. They are driving the private sector into the public core of the NHS, offering up essential emergency provision to the lowest bidder.
"Whistleblowers have contacted Labour with concerns that even the most serious 999 calls are being handled by private ambulances without properly trained staff and equipment. This is cost-cutting privatisation at its crudest, with a real risk that patient safety will be compromised. Jeremy Hunt must provide urgent assurances about the quality and safety of these private services."
On Wednesday the House of Lords will debate controversial rules which could open the health service up to more competition.
Mr Burnham said: "This week, Labour will ask the Lords to call a halt to David Cameron's mad dash to privatise the NHS. He promised to let doctors decide but now is ordering them to open up the NHS to full commercial competition. The Prime Minister needs to be reminded that the British people have never given him permission to put their NHS up for sale."
A spokesman for Mr Hunt said: "Contracts to deliver patient transport are decided locally, and should be based on what is required to meet patient demand. As we know the NHS is seeing an extra one million more patients in A&E compared to two years ago and despite the additional workload it is coping well."
A spokesman for the Independent Ambulance Association said: "It is disappointing that politicians appear not to know that since April 2011 independent ambulance services have been regulated by the Government's Care Quality Commission and are legally required to comply with exactly the same rules as all NHS ambulance trusts in respect to the care, well-being and safety of patients.
"The fact is that patients are well served by the public and private ambulance services and there is certainly no evidence that they are at risk more by one or the other."