The Queen Elizabeth II Bridge at the Dartford Crossing was closed after a World War Two bomb was found nearby.
The device was discovered shortly before 9am on Tuesday and the Ministry of Defence bomb disposal team is on the way to the scene, Essex Police said.
Motorists were caught up in huge queues on the roads surrounding the bridge following the discovery.
The bridge has now re-opened but police said it could be closed again later so the bomb could be removed.
It comes after a 500kg World War two bomb was discovered in Thames near London City Airport on Sunday.
An Essex Police spokesman said: We are on scene at an industrial estate in Oliver Close, Grays after an unexploded World War II bomb was discovered.
We were called at around 8.55am today.
The Queen Elizabeth II Bridge was closed for a short time but has re-opened.
A Ministry of Defence bomb disposal team is on the way and the bridge may be shut again when they dispose of it.