An apparent increase in the use of private ambulances in the NHS is a risk to patient safety, Labour has warned.
It says freedom of information requests show spending on private vehicles by three English ambulance services rose by millions over two years.
Shadow health secretary Andy Burnham said the services were "being privatised without proper debate".
The government accused Labour of "rank hypocrisy", saying private ambulances were introduced by the last government.
The figures showed an increase in spending of £5.4m on private contractors by the South East Coast Ambulance Service, from £1.9m, in 2010/2011, to £7.3m, in 2012/2013.
In London, that figure rose by more then £3.8m, from less than £400,000, in 2010/2011, to £4.2m, in 2012/2013.
In Yorkshire the figure rose £1.3m, from £500,000 to £1.8m.
Mr Burnham is asking Health Secretary Jeremy Hunt for "urgent assurances about the safety and quality" of private ambulances.
He said people would be "stunned to learn that even blue-light 999 services are being privatised".
"It is proof that the coalition sees no limits on privatisation in the NHS," he added.
"They are driving the private sector into the public core of the NHS, offering up essential emergency provision to the lowest bidder."
He said Labour had evidence from whistleblowers 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."
Referring to a House of Lords debate taking place on Wednesday over controversial rules which could open the NHS up to more competition, Mr Burnham said peers were being asked "to call a halt to David Cameron's mad dash to privatise the NHS".
"The prime minister needs to be reminded that the British people have never given him permission to put their NHS up for sale," he said.
A spokesman for Mr Hunt said contracts to deliver patient transport were 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.
"Using a variety of healthcare providers to deliver patient transport services is a system which was started under the last Labour government but is an approach they now criticise."
He said "this rank hypocrisy" showed that "the Labour Party is more concerned with playing party politics than meeting the needs of patients."
The trade body which represents private ambulance services, the Independent Ambulance Association, said patients are well served by both the public and private sector.
"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," it said.