The Society of Radiographers is calling for government action to stop the 'creeping privatisation' of NHS scanning.
An unpublished survey of representatives from 103 trusts found 83 per cent provided magnetic resonance imaging (MRI).
Of these, 45 per cent used private mobile scanners or companies to provide the service.
It also found that 47 per cent of trusts were planning to provide new MRI services. Of these, 21 per cent intended to use private mobile scanners or companies.
'These results were a shock to us,' said survey author Eleanor Ransom. 'We had anecdotal evidence that trusts were not using NHS staff, but we did not expect to find it happening on this scale.'
The survey found that 15 per cent of trusts planning new MRI services were intending to use a private company based on site.
The society is concerned that at least some of these schemes will be funded through the private finance initiative.
Ms Ransom said it would like to see the NHS Executive's PFI review conclude that scanning services should not be subject to PFI. If they are, then it wants regulations covering PFI deals worth more than 1m extended to all privately funded projects to guarantee union involvement.
A motion urging the government to 'ensure clinical services and staff are not 'hived off' to the private sector', will be debated at this month's Trade Union Congress.
The Society of Radiographers says the use of private sector companies could 'fragment' the screening service, undermine terms and conditions for NHS staff and fail to provide guaranteed standards for patients.
However, managers claim that many trusts cannot afford vital scanning equipment without using PFI.
The Mayday trust in Croydon, Surrey, signed a contract in April with Lister Bestcare for the company to finance, build and operate a 1.5m MRI centre on trust land.
According to corporate planning manager James Cardwell-Moore, it was the only way the trust could afford the centre.
'Our capital budget is only around 1.5-2m a year... we are simply not in the ball game for that sort of investment,' he said.
There is also confusion over the government's line on contracting out clinical support services.
Previously, ministers have spoken out against it, but an NHS Executive spokesman said the situation will remain unclear until the health PFI policy review is published. He could not say when this would be.
'The review will list services which can and can't be contracted out,' he said.
'New guidelines will be set which will probably come into effect next April.'