The Scottish Cabinet has backed plans to build a new bridge or a tunnel across the Firth of Forth.
Ministers have yet to decide on the route and funding method but the project is expected to cost about £1bn.
The cost to replace the existing Forth Road Bridge, which is suffering from corrosion, is expected to be almost double that of earlier estimates.
Ministers looked at expert assessments and agreed on Wednesday, in principle, to building a new crossing.
Transport Minister Tavish Scott said three plans had been drawn up as possible routes for the new crossing.
Mr Scott said: "The scale of the crossing, which will cost at least £1bn, will be the biggest construction project since devolution.
"Cabinet is clear, doing nothing is not an option given the importance of this crossing to the Scottish economy and the people of Fife.
"Work will continue to assist a final decision which will be made this summer on where the crossing will be, what it will be, and how much it will cost."
Alastair Andrew, general manager of the Forth Estuary Transport Authority (Feta), said: "We welcome the progress that has been made by Transport Scotland and the executive and we look forward to contributing our experience and expertise as the project progresses."
Fife MSPs and business leaders have been backing plans for a new crossing, although some campaigners say it is already too late to prevent major disruption.
Corrosion problems have come to light on the existing crossing
Final decisions on the funding method, the route and whether a bridge or tunnel would be best will be taken after the Holyrood elections.
Phil Flanders, of the Road Haulage Association, has also backed the construction of a new crossing as doubts persist over the present road bridge.
"Nobody seems to know just how long the bridge will have but we do know that, in the current state, HGVs will be banned from 2013," he said.
David Davidson MSP, Scottish Conservative transport spokesman, said that Labour had stalled on delivery for too long.
"As early as 1993, we announced that preliminary work on a new bridge and associated road links would begin," he said.
"Preliminary investigations into options for a new crossing began, and land was set aside for the project. Fourteen years later, what is the current situation?
"Make no mistake, Labour has been the roadblock to a new crossing for the Forth. They have had a decade to get the ball rolling. Instead, they have prevaricated for so long that the cost is now estimated at at least £1bn."
The SNP welcomed the decision, which they described as a Labour and Lib Dem "cave-in".
"It must be remembered, however, that it was not so long ago that Mr McConnell said it would be 'particularly stupid' to proceed with a new Forth crossing," said Fergus Ewing MSP, transport spokesman.
"Clearly they have now realised their failure to take a position on building a replacement crossing was untenable, and so they have once again adopted SNP policy."
Despite executive reports, the Scottish Green Party said it is not convinced that the existing bridge is beyond repair.
Mark Ballard MSP claimed that ministers did not have all the necessary information to make a proper decision and had not acted responsibly.
"The facts, not electioneering and pandering to the road lobby, should determine policy," he said.
"If the studies show that the existing bridge cannot be repaired, a replacement will of course be needed, but having two road bridges open will simply increase pollution, worsen congestion between the bridge and Edinburgh city centre and divert money away from public transport alternatives."
There are concerns that a new crossing will increase congestion
Instead Green MSPs are arguing for investment on traffic reduction measures, such as the creation of new rail stations in Fife.
Friends of the Earth Scotland Chief Executive, Duncan McLaren, also raised concerns over a lack of evidence.
He called for ministers to commit to ensuring any new crossing would have smaller capacity for private vehicles but enhanced capacity for public transport.
The South East of Scotland Transport Partnership (Sestran) suggested that the new crossing should have lanes that prioritised public transport and HGVs.
Chair of Sestran, councillor Russell Imrie, said the promoter of the new crossing should be required to "ensure that levels of traffic entering Edinburgh will be no higher - and preferably lower - than would have been forecast without an additional crossing".