Closure is expected to have a far-reaching impact in the run up to Christmas, such as on shopping and festive leisure trips to Edinburgh, as well as for fans travelling to sporting events.
The economic impact figure was calculated using past analysis by the Scottish Chambers of Commerce and bridge officials of the cost of travel delays alone, when replacement of the bridge’s main cable was considered in 2008. The chambers said a year-long closure would cost £1 billion.
The full cost of closure until around 5 January is still being calculated by the Scottish Government, but transport minister Derek Mackay said delaying it could have cost even more.
He announced yesterday it would take a week to prepare the work, on a broken section beneath the southbound carriageway, and two weeks to complete.
Mr Mackay described the incident as a matter of “national significance”, while officials urged travellers to re-consider whether their journeys were necessary.
The closure decision was taken by ministers on Thursday night after further inspections of the damaged section of the girder framework which supports the carriageway near the north tower.
A contraflow on the northbound carriageway was imposed when the fault was spotted during a routine inspection on an adjacent component on Tuesday night, with a lorries banned from the bridge on Thursday afternoon.
The damaged area was last inspected in May and is normally checked annually.
Mr Mackay said: “Action now will mean that any closure is much shorter than it might be if we waited.
“We are aware of the potential economic impact, for strategic traffic in the east of Scotland and on people living in local communities.
“This is an unprecedented challenge in the maintenance of the Forth Road Bridge.”
Engineers said further damage to the section could have caused the bridge deck to drop by around 6in and would have led to the closure of the crossing for “a number of months”.
Labour backed the move but the Conservatives described the situation as a “scandal”.
Transport spokesman Alex Johnstone said: “Although the repairs are necessary for the safety of the public, the fact that the decision has been taken to close it for so long shows a complete lack of forward-thinking by Derek Mackay and the SNP Government.
“The closure will have a devastating impact on Scotland’s economy, not to mention the many commuters who rely on the crossing for their livelihoods.”
Scottish Liberal Democrat leader Willie Rennie said: “This extended closure will cause huge inconvenience and impact on families and businesses alike.
“We also need to know how we have ended up in a situation where we have been forced to close the bridge for weeks on end while extensive repairs are carried.”
Chris MacRae, head of policy for Scotland for the Freight Transport Association, which represents hauliers, said: “This has come at the busiest time of year for freight operators and will cause serious issues in terms of delivery times and drivers’ hours over the Christmas period.”
The Federation of Small Businesses also expressed alarm.