The operators of the Forth Road Bridge have voted in favour of building a new bridge to cope with increasing traffic.
Thousands of cars cross the Forth Road Bridge every day
They made the decision even though the Scottish Executive has said it has no current plans to fund the crossing.
The Forth Estuary Transport Association - which is made up of neighbouring local councils - unexpectedly backed a new road bridge by six votes to five.
The board's decision was condemned by environmental groups, who have urged ministers to intervene.
Feta approved the most expensive strategy to improve the flow of transport across the Forth on Friday.
Package of measures
The projected costs are £700m for a new bridge which would take 10 years to build.
A further £300m for a package of measures was approved to ease congestion on the current crossing over the next few years.
The Scottish Executive said it would be two years before ministers looked again at the possibility of funding such a project.
A spokesman said: "We will wait and see the finalised local transport strategy they may submit.
"We have no current plans to build another crossing on the Forth."
Feta's decision was quickly condemned by environmental campaigners.
Friends of the Earth Scotland's head of research, Dr Dan Barlow, urged ministers to step in and stop the "crazy" idea in its tracks.
He said: "A new road bridge is the last thing that is needed if Scotland is serious about tackling congestion and cutting climate pollution.
"By choosing to ignore the advice of their own officers and back a new road bridge it has become clear that Feta is totally out of control."
Colin Howden, campaign manager for Transform Scotland, said Feta had rejected recommendations to adopt a sustainable package of measures.
He called for the body to be wound up - and urged Transport Minister Nicol Stephen to intervene personally and block the proposals.
But Feta chief executive Douglas Sinclair said the new crossing was "the only solution".
"We are committed to a series of measures to alleviate traffic congestion crossing the Forth," he said.
"But there is clear evidence from the consultants that even if you implement all these measures the bridge will still not cope and it will not compensate for the year-on-year growth of traffic."
He said the congestion problems would continue to mount despite the 10-year "breathing space" these measures would provide.
"The board took the decision that Feta had to commit itself today to building the new multi-modal crossing as nothing would be gained by delaying making this commitment," he said.