Bracknell Forest Council is urging motorists to take extra care at the roundabout junction, near Sainsbury's and the Hilton Hotel, while engineers work to fix the problem.
A statement from the council said: "In the interests of public safety, following this evening's rush hour, a temporary road layout will be put in place. Drivers will not be able to drive straight through the junction on the A322 approaches, and instead they will be diverted around the outside of the junction.
"We apologise in advance for any inconvenience this may cause, and once again urge motorists to take extra caution at this busy junction."
The cause of the outage has not yet been established.