The main road between Lydney and Whitecroft was closed today (Monday) for the start of emergency investigation works.
The closure of Forest Road is near the entrance to the Dean Forest Railway centre at Norchard and will be in place for about a fortnight.
The closure is to investigate a possible landslip underneath the road and cracks to the surface.
The official diversion route is through Bream, Coleford and Broadwell.