The move, announced at 2pm today with immediate effect, was followed by confirmation that other traffic would continue to be reduced to one lane each of the northbound carriageway during this afternoon’s rush hour.
The official Traffic Scotland travel information service tweeted: “*ATTENTION* @forthroadbridge - Weight restriction in place for all vehicles over 7.5 tonnes MUST use alternative route.
“Bridge team confirm contraflow will be in place through evening peak. AVOID the #FRB if you can.”
The setback came after bridge officials said they hoped the traffic restrictions could be at least partially lifted this afternoon.
They also warned of likely further disruption during tomorrow afternoon’s rush hour because of strong winds.
High-sided vehicles may be banned from crossing around noon, with cars only allowed on the bridge from around 3pm, when gusts could top 70mph.
The bridge may also close to all traffic around midnight when winds are forecast to increase to 82mph.
The southbound carriageway has been shut since Tuesday evening when part of its support-girder framework was found to be broken, in a regular inspection.
The latest development confirms The Scotsman reporting that restrictions may have to continue until a temporary repair is made, such as banning heavy vehicles.
Restrictions may also be necessary during the work, which could take several days.
The Freight Transport Association, which represents haulage firms, expressed alarm.
Head of road network policy Malcolm Bingham said: “We are very concerned about the situation on the bridge, and understand from [Scottish Government agency] Transport Scotland that they are working to resolve the problem.
“It is imperative we understand fully what is happening on this vital piece of infrastructure, and therefore are seeking further details so that we can keep our members up-to-date with any changes and they can then can make informed decisions.”
Tailbacks of three miles during today’s morning rush hour and more than one mile on either end of the bridge this afternoon came after traffic was reduced to one lane in each direction as a safety precaution.
ScotRail said it had been unable to add extra carriages to trains across the Forth Bridge because it had none available.
Vehicles are sharing the northbound side after a “defect to a steel component” was spotted in a routine inspection of the southbound carriageway.
Bridge officials said the restriction was imposed primarily to prevent further damage by the weight of traffic.
The defect is the first of its kind to be found on the 51-year-old bridge, and is thought to have happened recently as the network of girders is regularly inspected.
A bridge spokesman said: “Engineers and specialists carried out a more detailed inspection yesterday morning and are currently in the process of completing calculations with a view to implementing a temporary solution in the near future and a permanent repair at a future date.
Engineers had to edge along a 2ft-wide girder to reach the fault, which is at the bottom edge of the “truss”, or carriageway support framework, 30ft below the bridge deck and around 130ft above the water.
It is also in a very exposed location on the east side of the bridge, beside the north tower.
A spokesman for the bridge, which is run by contractor Amey for the Scottish Government’s Transport Scotland agency, said: “This is not something we have seen before, and we can’t take chances with safety.”
Southbound traffic queued yesterday on the M90 back beyond Halbeath, east of Dunfermline, while there were also tailbacks around the Kincardine Bridge.
Abnormal loads have been temporarily banned from the bridge. The footpath and cycle tracks are closed, with patrol vans transporting cyclists who need to cross.