Scottish Finance Minister John Swinney denied yesterday that the idea of building a tunnel under the Firth of Forth had already been abandoned in favour of proposals for a new road bridge.

Mr Swinney's comments came as he revealed details of a public information exhibition setting out proposals for a replacement Forth crossing.

The exhibition includes two types of road bridge ranging in price from £1.5bn to £1.7bn and three types of tunnel, costing between £2.1bn to £2.3bn.

Engineer John Carson, who leads a campaign group fighting for a road tunnel under the Forth, claimed that Transport Scotland, the national transport agency for Scotland, had already decided to build a new bridge and the public information exhibition this month was "window dressing."

Mr Carson said: "It seems to us that Transport Scotland has already made up their minds to build a bridge and dismiss the tunnel idea, despite the fact it has many advantages and would be more environmentally friendly. This public consultation exercise is quite biased against the tunnel plan."

Mr Swinney, speaking in North Queensferry, said that improving transport infrastructure and investing in public transport was critical to achieving sustainable economic growth and protecting Scotland's environment.

He denied any decision had already been made and said a final decision as to whether a tunnel or bridge would be built, and how it would be financed, would be made by the executive in the autumn.

Both the new bridge and tunnel plans are to the west of the existing Forth road bridge. No decision as to what would happen to the old road bridge and when it would be taken out of service has yet been made.

When the Forth road bridge opened in 1964 it was the longest suspension bridge anywhere outside the USA. At that time around two million vehicles used the crossing.

In 2006, this figure was close to 12 million, growing more than five-fold in 40 years, far higher than the national average traffic growth.

The bridge is now showing signs of deterioration, mainly as a result of the increasing number of vehicles but also due to weather and climate.

Transport Scotland had looked at ongoing repairs which included replacing cables but carrying out these works while the bridge remained open would have taken up to seven years and there could have been contraflows for 56 weeks and partial closure for 48 weeks over a four-year period. Because of that, Transport Scotland and a group of leading transport consultants explored options for a new Forth crossing and have worked down from 65 options to the final shortlist.

Mr Swinney said: "The Forth replacement crossing will be one of the biggest infrastructure investments ever seen in this country.

"It is vital to Scotland's future, and with such an important decision I am very keen to hear people's views on the proposals.

"If we are to keep Scotland moving in years to come, and achieve our aim of improving economic growth, it is clear that we need the public to help us on more than just the decision on the type of crossing.

"Today's reports show that we face growing traffic levels on our roads.

"The Scottish government will continue to do its bit and we will continue to invest in infrastructure that tackles congestion, improves connectivity and reduces journey times, and we will continue to encourage people out of their cars.

"We must give people the choice of excellent public transport. We must encourage change in the mode of transport used by the public to protect our environment and ensure our economy can develop without being constricted by an increasingly congested roads network."

Meanwhile, work is continuing on the new Kincardine bridge across the Forth which is being built to relieve traffic congestion. Traffic heading for Fife will use the original Kincardine Bridge while vehicles going to Clackmannanshire will use the new crossing, due to be completed in 2008.