The M48 was closed westbound until 2pm as there were not enough people to man the tollbooths – leading to long delays.
It meant motorists heading away to enjoy the heatwave this weekend were diverted on to the M4 Second Severn.
The road is used by around 17,000 drivers each day and charges £5.60 to get into Wales.
The Highways Agency said it had no choice but to close the tolls to protect the safety and staff of drivers.
A spokeswoman for the Highways Agency told the BBC: "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 applied to all westbound lanes between Junction 1 and 2 on the M48, which links Chepstow in Monmouthshire and Aust in South Gloucestershire.