The A3 has been closed in both directions at the Hindhead Tunnel in Surrey.
Emergency repairs are being carried out within the tunnel itself, which has led to both carriageways being shut to traffic.
National Express is warning customers its services from Portsmouth to London, the 030 and 031, will be affected.
It is understood the closures will be in place into the night.
A diversion in operation – northbound traffic is advised to follow signs showing a solid diamond symbol, following the A325 northbound and A31 eastbound, to rejoin the A3 at Guildford.
Motorists travelling southbound are advised to follow the signs showing a solid triangle, which follows the same diversion in the other direction.