ONE lane of the road between Crickhowell and Gilwern has been closed due to fears the bank underneath could collapse.
Traffic lights have now been put in place on a 50 metre section of the A4077 between Crickhowell and Gilwern.
Powys County Council said its highway inspectors found slope instability within the adjacent bank below the road.
The council and the Canal and River Trust are co-ordinating investigation works into the cause of this instability, which are expected to last into the early autumn.
Councillor John Brunt, cabinet member for highways, said: "We would like to apologise for the inconvenience this will cause but the lights are needed in the interest of road safety.
"In the meantime, remedial drainage work will be carried out to reduce the stress on this section of the road while we wait for the results of our joint investigation."