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An "unprecedented" level of staff sickness has led to the closure of the M48 Severn Bridge westbound.
The Highways Agency said it opted to close the tollbooths on the bridge to protect the safety of staff and drivers.
Motorists are being advised to travel via the second Severn crossing, now known as the Prince of Wales Bridge, instead.
The closure is expected to remain in place until 14:00 BST on Saturday.
A spokeswoman for the Highways Agency said: "This is an unprecedented level of staff sickness.
"If we haven't got staff members in crucial roles on the toll plaza it is not safe for our team or the drivers so we have taken the decision to close the tolls.
"We are telling motorists to use the other bridge.
"Staff have been diverted to the second Severn crossing to assist staff there and keep traffic flowing."
The closure applies to all westbound lanes between Junction 1 and 2 on the M48, which links Chepstow in Monmouthshire and Aust in South Gloucestershire.
It remains open in the eastbound direction, which is not controlled by tolls.
The Grade I-listed bridge, which originally formed part of the M4 until the Second Severn Crossing opened in 1996, carries about 17,000 vehicles per day.