Research shows England’s ambulance trusts spent more than £92m in the last year on private ambulances and taxis to transport patients.
Some trusts said they rely on private ambulances due to a chronic shortage of NHS staff and ongoing problems with recruitment.
Earlier this year, the Care Quality Commission (CQC) published a damning report warning that patients were being put at risk by private ambulances.
It found that some firms were failing to obtain references or carry out criminal records checks while a lack of staff training was leading to serious patient harm.
For the latest study, data was obtained from the 10 ambulance trusts in England via the Freedom of Information Act.
It detailed how much they spent on private ambulances and taxis, and whether these were used for 999 calls or non-emergency patient transfers.
The Yorkshire Ambulance Service (YAS) increased its spend on private ambulances to £4,906,384 in 2018/19, up from £3,343,771 the year before.
Private ambulances were used to respond to 3,606 calls to 999 in 2018/19 (0.48 per cent), up from 2,511 on teh previous year (0.35 per cent).
John McSorley, a divisional commander with YAS, said: “We respond to the overwhelming majority of our emergency patients using our own staff and vehicles. However with continuing high levels of demand, we do occasionally use private providers; in the majority of cases this is to support us when patients need transferring between hospitals. The amount spent on this accounts for a relatively small percentage of our overall workforce budget for A&E Operations.
“We are committed to increasing our A&E Operations workforce and have recruited an additional 400 staff to A&E Operations in 2018-19. We have robust safeguards and operating standards in place to ensure that the use of private services maintains our own expectations of the level of care provided to patients.”
The data showed that NHS ambulance trusts in England spent £92,476,915 on private ambulances and taxis in 2018/19.
This was up on the £90,862,230 the year before.
A statement from the Independent Ambulance Association (IAA) said: “The support provided by independent ambulance providers to NHS ambulance trusts is vital in ensuring that the emergency needs of patients are met, particularly where there are peaks in demand.”
It said private providers must be registered with the CQC and are “subject to additional and rigorous checks by NHS trusts.
“Our view is that monies spent on independent ambulance providers by NHS ambulance trusts are an investment to ensure the highest standard of care is provided; it is also very cost-effective."
The Government says it has invested millions into trusts so they can buy new vehicles and boost performance.
A Department of Health and Social Care spokesman said: “We are committed to a world-class NHS free at the point of use and the proportion of spend on the independent sector in the last financial year is at the same level as 2014/15.
“With record numbers of paramedics in the ambulance service, we are supporting them to deliver the response patients need in an emergency, including investing more than £3m for trusts to buy 256 new vehicles and introducing improved performance standards.”
However, Shadow health secretary Jonathan Ashworth said: "Labour have longed warned against the risky, wasteful practice of privatising patient transport services.
"Matt Hancock promised 'no privatisation on his watch' and yet just in the last few weeks another patient transport service in Worcestershire was privatised, leaving staff in tears and fearing for their jobs.
"Patients are utterly fed up of profiteering companies like this taking our NHS for a ride.
"Labour will end the Tory privatisation racket and put patients, not profit, first."
Unison national ambulance officer Colm Porter added: "Spiralling costs for private ambulance hire are siphoning tens of millions from squeezed NHS budgets that would be better spent elsewhere.
"Research suggests private firms are cutting corners and failing to provide the level of care needed for patients.
"The NHS sorely needs investment to address staff shortages and stem the flow of departures from the ambulance service, rather than papering over the cracks."