Health bosses have defended spending almost £7m on private ambulances last year – double the amount from 12 months before.
The North West Ambulance Service (NWAS) said it is being forced to hire outside vehicles to cope with spikes in demand, particularly on bank holidays and during extreme weather.
NWAS said it has now been given extra money to recruit more staff and buy more vehicles, but has warned that will take time.
In 2014-15, £3.2m was spent on hiring ambulances from private firms and charities such as St John Ambulance and the British Red Cross. That soared to £6.8m over 2015-16.
Kevin Lucas, from Unison, said the service needs to train more paramedics and ambulance staff to stop them looking elsewhere for cover.
He said: “The simple fact is they’re not able to train enough staff because the role is not as attractive as it used to be. The physical and mental pressures on staff are increasing year on year, that’s what our members are telling us. And yet their pay has not been reviewed in 10 years.”
Earlier this year, paramedics from Poland were recruited to plug gaps.
NWAS interim director of operations, Ged Blezard, said the number of urgent, life-threatening calls has rocketed but private vehicles were only sent to less serious calls.
Mr Blezard said: “Purchasing new ambulances can take up to 12 months and recruiting enough new staff can take up to three years.
“To fill that gap in the meantime, the trust increased its use of private and volunteer ambulance services and organisations. We hope to see that figure decrease once we have sufficient resources for demands placed upon us.
“While the private services we employ are manned by staff, who are subject to the same safe-guarding checks as NHS staff, the trust only sends these to the lower level, non life-threatening incidents, enabling us to free up our own staff for the more urgent cases.
“We do have challenges to contend with, such as a national shortage of paramedics. To combat this, the trust has improved the internal process for existing frontline staff to become qualified paramedics and has also improved the external university route for those wishing to become paramedics.”
“Using private ambulance services for expected periods of high demand such as Bank Holidays, Christmas and during extreme weather is something the trust has done regularly and will continue to do so should the need arise.”