More than half the contracts for the NHS to provide patient transport services are at risk of being given to private or voluntary-sector providers, ambulance trusts have warned.
A risk assessment by England's 13 ambulance trusts found more than half of their current patient transport work is at risk of being delivered by other providers.
Chief executives have pledged to resist future threats to contracts - and try to win back those lost. The contracts the trusts have retained are worth£180m.
The figures were compiled by the National Ambulance Partnership Forum, which brings together all trust chief executives and England's three biggest unions, Unison, the GMB and T&G.
Unison national ambulance officer and forum member Sam Oestreicher said members had concerns about how closely outside providers were monitored.
There were workforce implications too, he said.
'Often the paramedics of tomorrow come from the patient transport services of today. If they are contracted out you lose that career pathway,' he explained.
Mr Oestreicher claimed the services were commissioned 'normally solely on cost rather than quality or patient care criteria'. The loss of contracts would deprive trusts of vital income, he added.
GMB lead for the forum Sharon Holder said there is 'not enough evidence to show the service has improved' where it has been contracted out.
'In a strategic policy sense we haven't thought about the best use of patient transport services at all. If we had we would have had a better system set up rather than seeing it as the weakest link which can be contracted out,' she said.
A report by the forum states: 'The partners at [pensions organisation] NAPF will be looking at areas where a joint, national approach will assist trusts locally in resisting future threats and winning back some of the contracts that have been lost.'
One of the most recent contracts to be lost by the ambulance service came in February when a consortium of trusts in Bedfordshire and Hertfordshire struck a deal with national patient transport firm Medical Services.
The consortium's implementation group chair Roger D'Elia said the ambulance trust 'will now be able to focus its resources on continuing to provide emergency blue-light services, which are among the best in the country'.
A Department of Health spokesman said the onus was on commissioners to monitor the standard of patient transport services. The DoH hoped to introduce accreditation and, with the Healthcare Commission, national standards for independent providers in the future, he said.