A union has raised concerns about patient safety after it emerged that workers from a private ambulance firm were staying at a campsite between shifts.

Questions have been raised after it emerged that staff from ERS Medical had been sleeping in tents at a Cambridgeshire site when travelling out of their area for work.

The company, which has a contract with the East of England Ambulance Service, defended the decision taken by six workers, who have been staying at a campsite near Huntingdon because that was all they could afford on their £35 daily allowance.

However, officials from the GMB union, raised their concerns about the news and called for the private firm to make alternative accommodation arrangements for its staff or employ more workers in the local area.

Tony Hughes, GMB organiser for ambulance staff, added that the issue highlighted the East of England Ambulance Service's overreliance on private contractors.

'The ambulance service has steps in place to hire more staff over the next 12 months. However, they are still using a lot of private ambulances. The company has agreed a price with the East of England Ambulance Service and they have to fulfil the contract. Staff staying in a field is not really ideal.'

'There are hygiene and infection issues and change of uniform issues. It raises lots of questions. The last thing you want are people coming to your aid who are exhausted,' he said.

ERS Medical, which has bases across the region in Norwich, King's Lynn, Bury St Edmunds, and Papworth, insisted that its six staff at the Cambridgeshire campsite were happy with the arrangement.

A spokesman said: 'We always provide suitable financial support to ensure that all crews have sufficient rest between shifts. We have specifically questioned our crews as to the suitability of the campsite and have received unequivocal responses that say it's a great site with appropriate facilities and that they enjoy good sleep and comfort.'

Denise Burke, Labour prospective parliamentary candidate for North Norfolk and of the Act on Ambulances campaign, added: 'The ultimate responsibility rests with the East of England Ambulance Service. If they are going to commission services, they have to monitor the contracted service and make sure it is good enough.'

From April 2011 to August 2012, the East of England Ambulance Service shelled out £12.8m on private ambulances, figures released under the Freedom of Information Act show. The trust has pledged to bring down its private ambulance costs by £500,000 a month under its turnaround plan, which was launched in April.

A spokesman for the ambulance trust said: 'The quality of our service delivery is of upmost importance so before working in partnership with us, private ambulance organisations must undergo a thorough trust-accredited process and meet stringent criteria.'