The M25 clockwise is shut after a crash involving two cars and a lorry near J20 at Kings Langley.
Two air ambulances landed at the scene as reports say that two people were left seriously injured.
The M25 is closed clockwise between J19 and J21 while police investigate what happened.
Normal traffic conditions are expected between 8.15pm and 8.30pm.
Highways England have announced on Twitter that in the tailback a double decker bus has broken down anticlockwise between J21 and J20.
Drivers are urged to avoid the area as delays are expected.
Picture courtesy of Highways England