The clockwise carriageway remains closed between junction 3 for Swanley and the M20, and junction 4 for Orpington.
Traffic is queueing back to junction 2 for the A2 at the Darenth Interchange.
Emergency services, including Kent Police, are at the scene, with National Highways traffic officers managing the closure.
Drivers are being warned to expect delays of up to 45 minutes.
A diversion is in place.
Motorists should leave the M25 at junction 3, follow the A20 towards London, take the A224 at Crittall’s Corner and rejoin the motorway at junction 4.
AA Maps reports that Crockenhill Road bridge was hit by a vehicle, with congestion building as drivers follow the signed diversion.
National Highways is urging drivers to allow extra time, plan ahead, or delay their journeys if possible.
We have contacted Kent Police for more information.
