Launching a new report, public-sector union Unison stepped up its campaign to force the government to pull out of its commitment to the controversial procurement tool.
The long-term PFI critic published new research that it claimed proves that taxpayers' money is 'wasted' when the initiative is used.
The union insisted that the reports 'nailed the claim' from the government that extra costs of PFI were offset by increased efficiency.
It also said studies used by the Treasury on the cost of PFI programmes were 'fatally flawed' and not credible.
Dave Prentis, Unison general secretary said: 'Unison has always argued that PFI is a waste of taxpayers' money.
'This report knocks out another of the government's chief arguments for its continued use, namely that it generates value for money by improving the efficiency of construction procurement.
'It shows that the evidence used to justify this claim was not only selective but fatally flawed.
'The Government has relied on this 'efficiency' argument to such an extent that it has forced non-PFI projects to add up to 24 per cent extra onto their bids, to take account of notional cost and time overruns.
'The effect of this adjustment is to make non-PFI schemes look even more expensive and therefore less attractive.
'It is time for the government to say enough is enough. PFI is a discredited system and should be stopped right now, before more taxpayers' money is wasted,' Prentis added.