THE SNP leadership is backing the idea of building a tunnel under the Firth of Forth, instead of a new bridge.
The Scotsman has learned that the nationalists now believe a tunnel is the best option, and although the party has yet to make a final decision, it has all but ruled out a new bridge in favour of the underwater route.
The nationalists believe that a tunnel would be cheaper, at about 480 million rather than the estimates of anything up to 1 billion for a new bridge, and they say it would be better for the environment, more reliable and be cheaper to run.
The SNP also want to finance the new crossing with a bond issue, selling bonds to the public and to institutional investors to raise the capital for the project.
Alex Salmond, the SNP leader, believes bond issues should replace the controversial Public Private Partnership scheme for major capital projects and the Forth crossing would be ideal for the first one.
Following a successful Scotsman campaign, the Scottish Executive has given its backing to a new crossing over the Forth to replace the Forth Road Bridge.
The existing bridge is expected to deteriorate so fast that it will be closed to lorries in 2013 and closed to all traffic by 2018.
But ministers have yet to decide whether to go for a new bridge or a tunnel. They are waiting for the results of engineering and technical reports before making that decision.
The SNP, however, has no such reticence. Fergus Ewing, the transport spokesman, said: "On the face of the argument, a tunnel may have several advantages to a new bridge. Not only would it appear to be cheaper to build, but the experts say it could be completed more quickly than a conventional bridge and may even have some environmental advantages."
Mr Ewing also backed the issuing of bonds to pay for the new crossing.
He explained: "Financing the scheme through a bond issue under the SNP's proposed Scottish Futures Trust is seen by experts as significantly cheaper in the longer term than Labour's preferred option of private finance. While a replacement crossing is vital, we must ensure that we use a cost-effective method to fund the project."
However, the SNP's approach was condemned by Tavish Scott, the transport minister.
He said: "It is ridiculous to come out in favour of a tunnel at this stage. You have got to wait until you get the engineering reports."
Because of the housing developments along the shoreline, and because tunnels need more land on shore than bridges for the gently sloping entry points, a tunnel would have to be built further west, possibly between Pattiesmuir in Fife and the M9 at Old Philipstoun, than a bridge.
John Carson, an engineer and a South Queensferry resident, said that time was key and the tunnel could be built much quicker than a bridge.
He said: "You physically cannot build a suspension bridge in less than six years, and with the planning time involved, it would be difficult to build a bridge before 2018 [which is when the existing bridge faces closure]."
He added: "A tunnel could be built in three and a half years."
The Forth Estuary Transport Authority, however, is in favour of a new bridge rather than a tunnel because it could be based at Queensferry and feed directly into the existing road network.
THE SNP wants to finance a new Forth crossing by using a bond issue.
It would be called something like the Forth Tunnel Bond and anyone, anywhere would have the chance to purchase the bonds to finance the scheme. The government would underwrite the bonds and fix a rate of interest, making them a worthwhile investment. Bond-holders would be able to cash in their investments at maturity, possibly after ten years.
Bond issues are relatively common abroad, but have not been used in the United Kingdom to finance any major infrastructure projects for many years.