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Contractors and unions are locked in a political battle over the future of thousands of health workers that is threatening the viability of up to eight private finance initiative hospitals.

The dispute centres on whether ancillary workers such as porters, caterers, cleaners and security guards should transfer to the PFI consortium or remain with the hospital trust.

The hospitals hit by the dispute include a £260m scheme in central Manchester and the seven "third wave" projects. Future PFI schools are also likely to be jeopardised.

Unison, the union that represents many of the 15 000 workers affected, says they should not transfer to the PFI consortium. At the very least, it wants hospital trusts to ascertain whether transferral would really give better value for money.

The union is lobbying Alan Milburn, the chief secretary to the Treasury, who ruled in June that ancillary workers should not automatically transfer, and who is understood to sympathise with its view. The union is also likely to be supported by new guidance from the NHS Executive showing how a value-for-money comparison might be made.

But PFI contractors fear that, without the ability to manage the workers, the financial viability of PFI deals will be threatened. They are amassing evidence showing that many of the savings from the PFI derive from design innovations and facilities management directly related to ancillary services. Treasury officials are believed to be sympathetic to this argument.

Promoters of the Central Manchester scheme, due to be advertised next month, are also understood to fear that keeping the 800-1000 ancillary workers at the hospital will threaten its viability. Local Unison officials were due to meet hospital managers this week to press for assurances that the workers would not be forced to transfer.

Steven Weeks, a Unison national officer, said: "We believe that all the trusts should comply with the government's new rules, even if it means delays to schemes." Weeks said Unison had assurances from two "third wave" trusts – South Derbyshire and St George's in south London – that they will not transfer their ancillary staff. It is now seeking similar responses from the other five; Leeds, Oxford, Havering, Portsmouth and Blackburn.