Police Scotland, Amey, the Forth Road Bridge and Traffic Scotland confirmed that officers will close the M90 and Forth Road Bridge in both directions.
The road is due to close from 4.45pm to 6pm. All approaches will also be closed. The closure had been planned for after 6pm but this has had to change because of weather conditions.
This is following the discovery of materials believed to be used in the demolition process at the site of the new Queensferry Crossing.
Although there is no immediate danger to the public, a decision has been made to dispose of these materials with the assistance of the Explosive Ordinance Disposal Unit (EOD) and to close the road as a precaution during this time.
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The Forth Rail Bridge will also close temporarily while the controlled explosion takes place. Police are working with British Transport Police and Network Rail to facilitate this.
Chief Inspector Irene Ralston of Police Scotland said: “Exposed wires were discover today at the Queensferry Crossing, and while this is poses no immediate risk to the public, as a precaution we are compelled to close the Forth Road Bridge and the M90.
“We would advise motorists to avoid the area between 4.45pm and 6pm and seek alternative routes.
“We thank the public for their understanding at this time.”
Police Scotland will continue to update the public on Police Scotland Facebook and Twitter with developments, including information on any further road closures.