Huge queues built up around the Dartford tunnel after a motorbike crash.
Police had to close the right tunnel after a man and a woman came off their black and silver Suzuki GSX on the M25 near the entrance.
It reopened at 3.30pm.
The couple, from Romford, were taken to Darent Valley Hospital in Dartford.
The woman, who is in her 20s, has suffered serious injuries which were at first thought to be life threatening.
Essex Police were called at 12.25pm and congestion had backed up along the M25 towards the junction with the M20.
The queues were so long the Highways Agency took the decision to suspend the toll charges for drivers travelling northbound to help traffic pass through the toll booths quicker.
A Highways Agency spokesman added: "We hope this will reduce the risk of minor collisions in queuing traffic and we are doing all we possibly can on what is routinely a very busy period at the crossing."
Anyone with information about the accident is asked to contact Chigwell Road police station on 0300 333 4444.