Engineers found a crack in a key part of the Forth Road Bridge structure by accident.
An engineer from bridge operator Amey said his team had stumbled across the 20mm crack in a truss under the southbound carriageway while inspecting another part.
The discovery on Tuesday caused pandemonium, with massive tailbacks and lengthy delays, although the bridge was not fully closed in both directions until Friday.
Had the bridge not been completely closed for repairs, engineers warned the crossing would have dropped by around six inches, potentially shutting it entirely for “a number of months”.
Chartered engineer Mark Arndt, from Amey, said: “This is a complex engineering challenge.
“The component failure is in a difficult-to-access location and our response is also highly dependent on weather conditions. We continue to work around the clock on inspections, assessments and calculations along with the development of designs to effect the necessary repairs, while mobilising all the resources required to reopen the bridge as soon as is possible.”
Meanwhile, transport minister Derek Mackay said he took the decision to close the bridge amid fears keeping it open would cause “extensive damage” and put the public at risk.
He said it was hoped the crossing would reopen “as quickly as possible” but refused to rule out it remaining shut beyond the beginning of 2016.
“The decision to close the Forth Road Bridge is not taken lightly,” he said. “It is based on the expert opinion of the engineers who operate the bridge day to day and that of independent experts in the field.
“Every effort is being made to open the bridge as quickly as possible but safety is the main priority, however these works are weather dependent given the height and location of the bridge.
“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. On balance, following advice from engineers and independent experts, the full closure is essential for the safety of the travelling public and to prevent further damage to the structure of the bridge.
“The bridge operators Amey have a robust inspection team in place and these defects are problems that have only occurred in the last number of weeks.
“We are taking every step we can to lessen the impact of this closure.
“Action now will mean that any closure is much shorter than it might be if we waited.
“We continue to work closely with all partners to co-ordinate our efforts to lessen the impact of this closure. Additional bus and rail services are being provided between Fife and Edinburgh.
“Every effort and resource available is being deployed to repair the damage to the Forth Road Bridge and minimise the disruption to the public.”
The move will cause widespread chaos over the Christmas period, with hundreds of thousands of families and businesses facing lengthy detours and delays along the east coast.