The private finance initiative is helping to improve the quality of public sector construction work, parliament's spending watchdog has said.

The report on the role of PFI will come as a boost to Gordon Brown, who said on Monday that using the experience of the private sector helped to deliver better public services.

The National Audit Office found that Whitehall had generally obtained better price certainty and the on-time delivery of good quality assets using PFI contracts.

Data from the NAO showed that under the PFI only 22 per cent of public building projects had exceeded the cost initially expected by the public sector.

Under previous contracts up to 73 per cent had overshot the cost first budgeted for.

And the PFI construction projects were also being delivered on time, with just eight per cent being delayed by more than two months.

The NAO also found that most of the public sector project managers it surveyed were satisfied with the design, construction and performance of their PFI buildings.

"The theory is that PFI should incentivise the private sector to deliver good quality buildings on time and to the price agreed with the public sector," said NAO head Sir John Bourn.

"The results of our census show that this is being achieved in central government."

The findings provide support for the chancellor's argument that the PFI is "in the public interest".

"The public sector has always drawn on the expertise and experience of the private sector," Brown argued on Monday

"But, whereas in the public procurement of the past, private companies built and then walked away, PFI seeks to ensure that the companies involved are held transparently accountable for design faults, construction flaws overruns and long term maintenance so that value for money is achieved."

However, the Conservative chairman of the Commons public accounts committee, Edward Leigh, warned against claims that PFI contracts were always preferable.

"Greater certainty is not the same as better value for money. In our many examinations of PFI deals we continue to find mixed results," he said.