A new Forth crossing could cost up to £3bn, it emerged yesterday, after ministers gave approval in principle to a project which supporting documents say is likely to be tolled and privately financed.

The cheapest option, a bridge alongside the current road crossing, would cost at least £1bn at current prices, while a tunnel under one of the longer corridors would cost three times that.

Inflation in the overheated construction sector is running at twice normal levels, according to cost consultants Davis Langdon, which is likely to push the cost to at least £1.5bn by the target date of 2013 when deteriorating suspension cables could force restrictions on the existing bridge.

By contrast, it will be 2030 before there are question marks over the Victorian rail bridge, and then on grounds of capacity and not engineering safety.

Transport Minister Tavish Scott was indignant yesterday at suggestions that it was a cabinet decision based on the need for manifesto boasts. "A year ago you would have pulled us up for not making a decision," he said.

"We have been under enormous pressure from the Fife business community on this and for people to say a government should just do nothing is ridiculous."

But he also admitted the symbolism of yesterday's decision when he conceded that when the next two technical reports out of a series of five assessments are published a decision could be taken by the next government to drop the whole idea.

The SNP is on-side with the replacement, simply questioning why it has taken so long since capacity and structural issues arose with the existing crossing.

"It's been a long time coming for something the First Minister told me in 1995 was a stupid idea," said the SNP's Tricia Marwick.

Mark Ballard of the Greens called the decision premature: "If the studies show that the existing bridge cannot be repaired and is nearing the end of its life, a replacement will of course be needed, but having two road bridges will simply increase congestion and pollution and divert money away from public transport alternatives."

With three "corridors" being considered - upstream from Rosyth, just west of the current road bridge, or downstream from the rail bridge - Mr Scott said no decision had yet been made on whether the new link would be a bridge or a tunnel.

He would say only that the cost would be "at least £1bn". However, studies for Transport Scotland show this could rise to three times that for a tunnel under the longest point, between Dalgety Bay and the Hopetoun estate.

To put that £3bn figure in context, it is almost as much as the entire Scottish budget for infrastructure projects between now and 2012 across all modes of transport.

Three reports commissioned for ministers and published yesterday strongly suggest the crossing will be built under a public private partnership (PPP).

The most likely form of PPP will be a "design, build, finance and operate" scheme, with the supplier doing everything from the drawing board onwards, then maintaining the crossing for at least 30 years.

Tolls, already controversial on the existing bridge, are also being considered. Indeed, the reports name the likely sites for "toll plazas".