The NHS is spending over £1million a week on private ambulances for 999 calls, figures show.
Across the country, at least £61million is being spent annually on the extra provision amid warnings the government is failing to properly invest in ambulance services.
Unison said its figures were based on responses from two thirds of ambulance trusts in England that pay commercial companies to provide cover for critically ill patients.
More than a dozen private companies are being commissioned by ambulance trusts across England to fill widening gaps in services and to meet response times amid overwhelming demand, the union claimed.
Trusts are booking private emergency vehicles and crews up to a year in advance to be available to respond to emergency incidents such as road traffic accidents and stroke patients, said Unison.
It warned that spending tens of millions on private 999 cover is a "short-term fix, not a long-term solution to the crisis in ambulance services".
Speaking ahead of the union's annual health conference in Bournemouth, Unison's head of health Sara Gorton said: "This spend on private 999 services shows a lack of long-term planning and is a shocking waste of money.
"It's nothing more than a sticking-plaster solution.
"Ambulance services are in a desperate state because the Government has failed to invest long term.
"Patients are waiting ages for help to arrive or worse still dying before crews can reach them.
"Others are stuck in emergency vehicles outside hospitals for hours and hours on end waiting for a bed.
"This is a crisis of the Government's own making that can only be resolved with a long-term plan.
"Ministers must step up and come up with proper funding to tackle increasing demand and pay staff properly."
A Department of Health and Social Care spokesman said: "We are working hard to improve ambulance waiting times which have substantially reduced from the peak of winter pressures in December 2022.
"Our Urgent and Emergency Care Recovery Plan will allow people to be seen quicker by scaling up community teams, expanding virtual wards, and getting 800 new ambulances on the road. This is on top of £750 million we have provided this winter to speed up hospital discharge and free up beds."
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