Whitehall's spending watchdog has warned local authorities and government departments not to base private finance initiative projects on "spurious" figures.

The National Audit Office on Wednesday cautioned PFI planners to examine the wider benefits and risks of projects and not just the cost.

The watchdog claimed the government relied too much on "public sector comparators" - cost formulas that work out what projects such as new hospitals would have cost under conventional procurement procedures.

Deputy controller and auditor general, Jeremy Colman, said that many PFI measures relied on "pseudo-scientific mumbo-jumbo where the financial modelling takes over from thinking".

"It becomes so complicated that no one, not even the experts, really understands what is going on."

Colman said that in many cases there was scope to manipulate the figures and that the savings from PFI deals were often marginal.

"People have to prove value for money to get a PFI deal. But because that is wrongly seen to be demonstrated only by the public sector comparator, it becomes everything. If the answer comes out wrong you don't get your project. So the answer doesn't come out wrong very often," he said.

Colman said there were also wider issues that have yet to be addressed such as whether there is a sufficient transfer of risk and whether "people actually want a 30-year relationship with a supplier".

The report brought a strong response from public service unions, which have been vocal opponents of the government's PFI policy and are stepping up their opposition.

Unison published a list of PFI projects it claimed have already failed in Scotland.

The union also said that risk transfer was being used as an excuse to load extra costs on to the public sector comparator.

"Across all the public services there are failure after failure," said Dave Wastson, Unison's Scottish organiser. "The current dash for PFI schools is also going to be bad value for the taxpayer."

"All the projects we have seen add a 'risk transfer' cost to the public sector alternative which masks the cost difference between the public and the private sector. But the 'risk' doesn't get transferred. If the contractor goes bust or fails to deliver an adequate service, the authority picks up the tab."