The use of private ambulances in the east of England for 999 and non-emergency cases has doubled in the last 12 months.
East of England Ambulance Service NHS Trust (EEAS) spent £9,535,027 on private ambulances for 999 and non-urgent work in 2018/19, double the £4,791,155 the year before, figures show.
However the figure is still £5 million less than the £14 million it spent in 2016/17, an amount it blamed on staff shortages and rising demand.
The reliance on private ambulances to attend 999 calls rose, with more than double the number of incidents resulting in one being sent to the scene.
Some 26,428 incidents in 2018/19 involved a private ambulance being required, up from 12,947 the year before.
Private ambulances were sent to 5.21% of all 999 incidents in 2018/19, up from 2.59% the year before.
However spending on taxis for moving patients saw a tiny drop of £77 to £1,103,000 in 2018/19, compared to £1,180,000 in 2017/18
EEAS said it had hired hundreds of new staff but used private ambulances for overtime and spikes in demand, such as in winter.
It added: "It takes three years to qualify as a paramedic and we use private services to fill gaps in budgeted capacity whilst student paramedics complete their university studies and whilst we fill vacancies.
"Recruiting trained staff, particularly registered paramedics, is extremely challenging and whilst we continue to recruit and train a significant number of patient-facing staff, we continue to use private ambulance services so that we can respond to patients as quickly as possible and give them the best possible service."
The data, by the Press Association, found England's 10 ambulance trusts spent more than £92 million in the last year on private ambulances and taxis to transport patients.
Ipswich MP Sandy Martin said it was better any ambulance was sent rather than none at all, but he wanted to see EEAS improve its ways of working to ensure more were available.
"The bigger question is why they have to wait outside A&E doing paperwork," he said.
"EEAS needs to get its ambulances back out on the streets ASAP. The whole system of contacting the ambulance service, getting it to the right place and getting the paperwork done is where I think the trust needs to tighten up."
Unison national ambulance officer Colm Porter said spiralling costs for private ambulance hire were "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," he said.
"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."
The regulator found "ongoing issues with poor recruitment, training and safeguarding processes, with evidence of incidents of serious harm to people from staff that had not been properly recruited and vetted."
The Independent Ambulance Association said it played an important role in backing-up the NHS: "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."
A Care Quality Commission report in March warned patients were being put at risk from private ambulances.
The regulator found "ongoing issues with poor recruitment, training and safeguarding processes, with evidence of incidents of serious harm to people from staff that had not been properly recruited and vetted."
But the IAA said private providers had to be registered with the CQC and were "subject to additional and rigorous checks by NHS trusts".
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