The M74 extension plan is being scrutinised

If PPP is used, the cost of building the last five miles of the M74 could climb to between �750m and �1bn.

Scottish Green MSP Patrick Harvie said the admission was "astonishing" and would be "a waste of public money".

An executive spokeswoman stressed that no decision would be taken until a local public inquiry has concluded.

The scheme is intended to alleviate traffic congestion through Glasgow.

A billion pounds for the privilege of shaving a few minutes off a trip to the airport or the shopping centre is an absolute scandal Green MSP Patrick Harvie

It is expected the five-mile stretch of road will join the M8 near the city centre and if approved could be ready by 2008.

An inquiry is being held into the project at the Parish Hall in George Street, Glasgow, and is expected to last until the end of the month.

There are 370 objectors to the scheme including 41 from people whose businesses or properties are set to be compulsorily purchased.

A campaign against the road has been mounted by JAM74 (Joint Action against the M74), a coalition of community, environmental and sustainable transport groups.

Supporters of the extension believe it will create a main strategic link between Scotland and England.

'Best value'

The executive said a tender notice for the work contained a "design and build" option and a "design, build, finance and operate" PPP scheme alternative.

The PPP option would see an annual payment of between �25m to �34m paid to a private firm to build and maintain the road for a 30-year period - a total cost of between �750m to �1bn.

This cost is more than twice the maximum price currently being discussed at the public inquiry.

Under the PPP option the private firm would also be responsible for maintaining an additional 60 miles of motorway, the M74 north of Douglas in South Lanarkshire, the M75 and the M8 within Glasgow.

A campaign against the road extension is under way

The executive stressed that the PPP scheme had been suggested only to gauge interest from the construction industry.

"This notice seeks expressions of interest." the executive spokeswoman said.

"We are really trying to determine which option would provide the best value for taxpayers' money.

"No decision has been made. It will all depend on the outcome of the inquiry."

Glasgow MSP Mr Harvie described the potential cost as "astonishing".

'Waste of money'

He said: "We've been angry for a long time now about the �500m figure we'd heard of.

"But if this cost is going to double again it will be an unbelievable waste of public money.

"A billion pounds for the privilege of shaving a few minutes off a trip to the airport or the shopping centre is an absolute scandal, and I anticipate sheer outrage at this announcement."

Mr Harvie said the people of Glasgow had been "excluded, confused and misled" over the controversial proposals.

He said: "The new cost of this monstrosity is further evidence that the executive has not been honest with the public.