UNISON is calling for the “enormous” profits made by private finance initiative (PFI) companies to be made subject to a new windfall tax to provide “much-needed relief” for public services.

The union said the tax could be introduced if two amendments to the Finance Bill achieve enough support in the House of Commons on Wednesday.

Unison’s general secretary Dave Prentis said a windfall tax would be a “lifeline” to under-pressure public services and would force PFI firms to pay back some of the billions of pounds they have made “at taxpayers’ expense.”

He warned that PFI contracts were “threatening to overwhelm our already cash-strapped public services.”

“Local communities may now have shiny new schools and hospitals, but at huge cost,” Mr Prentis said. “The repayments on PFI debt have to be made first, and are so steep that teaching assistants are losing their jobs and patient services are being cut back.

"Meanwhile, the shadowy PFI firms just keep on hitting the jackpot, making millions more than they’d ever hoped to because George Osborne slashed their tax bill a few years back.”

John Lister, information director at campaign group Health Emergency, told the Morning Star the proposal was “definitely a step in the right direction” but said that the “long-term solution” was to bring the contracts back into public ownership.

He said: “I think anything that’s done to flag up the extent to which PFI is an extraordinarily expensive way of delivering hospitals is a good thing.”

But he added: “The problem is that a lot of these [companies] are now owned by offshore companies that don’t pay any tax at all, so what you have to do is tax some part of the body in this country, which is difficult to do.”

Mr Lister also said that the recent collapse of outsourcing giant Carillion provided an “opportunity” to make the case for public ownership of infrastructure.

He said the company’s demise “reopens the whole question about how these deals are put together [and] whether this is a sensible way of commissioning major public infrastructure projects.

“We should take this opportunity to bring them back into public ownership.”