An ambulance trust is spending five times as much on private ambulance firms as it did four years ago.
The East of England Ambulance Service uses the companies to transport non-emergency patients to hospital.
It said it had spent £3.99m on providers in 2010/11, compared with £790,000 in 2007/08, to free up paramedics attending emergencies.
The NHS trust rebutted concerns by unions about the level of training provided to the private crews.
Seven private companies are used across the region to help transport patients with urgent needs, such as for planned trips to hospital.
They can also act as "first responders" to provide first aid until a paramedic crew arrives.
The trust said of the 737,295 calls it received in 2007/08, 6,537 were attended by private crews.
In the year 2010/11, it took 823,213 calls, using 23,704 private crews
Director of service delivery, Alan Murray, explained the increase was a result of a change in emergency response time targets in 2008.
He said the use of private crews to assist with non-emergency work had helped meet this.
Tim Roberts from the union Unison believed the money would have been spent on the trust's own services.
He also raised concerns private crews did not receive the same amount of training as NHS employees.
He said: "We did some research earlier this year and found some of the private companies which the ambulance trust are using have a very limited training regime for their staff.
"We're wondering why it's appropriate for them to be cutting corners in this way and the ambulance trust allowing them to do that."
Mr Murray said the qualifications were similar to the trust's intermediate crews.
He added the companies were accredited by the Care Quality Commission and the trust undertook annual checks on them.
He added: "If we found a member of staff who didn't meet our standards we would ask the private company not to supply them to us in the future.
"If there were repeated failures by the company then we would stop using them."