There will be a second Audit Scotland probe

Presiding Officer George Reid said while such a move might be "attractive in theory" it is not possible contractually.

Mr Reid made the admission in a letter to the parliament's Finance Committee.

Meanwhile, Scotland's public spending watchdog has announced plans for a second investigation into the controversy.

The disastrous open-ended contracts signed in 1999 mean it has been open season for the Holyrood contractors SNP leadr John Swinney

The presiding officer said: "We are not, as you know, dealing with a fixed price contract.

"In that respect, the die was cast when the construction management route was chosen.

"The imposition of an overall cost cap, whilst attractive in theory, is simply not possible contractually."

Mr Reid said the corporate body would make every effort to complete the project within the latest �375m estimate.

Scottish National Party leader John Swinney said the presiding officer's letter was confirmation of Labour's "worst kept secret".

'Open season'

He said: "The disastrous open-ended contracts signed in 1999 mean it has been open season for the Holyrood contractors."

The Auditor General Robert Black, meanwhile, told the Audit Committee he plans to start his inquiry when the project, which is hugely over budget, is almost complete.

Mr Black carried out an investigation into the early stages of the project and a report was published in September 2000.

He said he would also assist Lord Fraser of Carmyllie in the independent inquiry set up by the First Minister Jack McConnell and Mr Reid.

Mr Black said his inquiry would not interfere with Lord Fraser's and would in fact assist in providing a full picture of what has gone wrong.

George Reid said a price cap was out of the question

He said: "I would imagine it would be perfectly possible to envisage a situation where (Lord Fraser's) inquiry and my investigation compliment each other and enable the parliament and the public to get a total picture of everything that happened around the project."

Mr McConnell announced last week that he and Mr Reid had invited Mr Black to help Lord Fraser with his inquiry.

Steadily rising

Mr Black said the details of his remit with regard to the inquiry had yet to be finalised, but he was "pleased" to assist.

"There is a large body of evidence already in existence and therefore at the very least I think it might be helpful to Lord Fraser if I assist him in understanding what is in that evidence to avoid any wasted effort," he said.

The cost of the new parliament building has been steadily rising since the project began and there was fury two weeks ago when it was revealed that it has now risen to �375m.