She said costs could increase as much as threefold by financial close. "University College London Hospital PFI rose from £120m to £430m. It's surprising that the National Audit Office hasn't even looked at this issue."
Pollock added that cost overruns were causing problems on PFIs in other areas. She said: "There was a £2.5m affordability gap for the PFI schools project in Haringey. The government bailed it out as it was a flagship scheme, but it can't bail out every scheme."
Public services union Unison, which has campaigned against PFIs, seized on the increases highlighted by the UCL research. National official Margie Jaffe said: "PFI doesn't prevent cost overruns – it institutionalises them."
Jaffe said: "PFI should not be seen as the only game in town. We want to see a level playing field to allow alternative procurement routes."
Pollock's claims were disputed by Lindsay Grist, director of the PPP Forum, a body representing PFI contractors, backers and consultants.
Grist said hospital trusts kept the same project specification during the first 18 months of a PFI bidding process in order to make it fair for all bidders.
Grist conceded that there were normally substantial changes required, and that they were introduced once a preferred bidder was named. However, she claimed that the resulting cost increases were rarely more than 10%, and increases of more than 20% would be unusual.
Grist said Pollock's figures were "plain wrong", and added: "I'd like to know where she gets them from."