It has been closed overnight since 16 May for "safety-critical repairs".
Motorists were urged to allow extra time for their journeys or consider alternative routes.
Problems with a safety system that stops over height vehicles entering the tunnel meant the tunnel could not be reopened on time, according to Andrew Broughton of Highways England.
Earlier, Highways England issued a statement which said: "There are long delays on the M25 in Kent anti-clockwise due to the closure of the Dartford Crossing east tunnel.
"There are also long delays on the A2 westbound between the M2 and the junction with the M25."