The 500 kg bomb was found near the town’s train station, which is in the western suburbs of Paris, just 48km from the heart of the French capital.
The discovery was made at 3.30pm and the police evacuated the area.
The bomb was 1m long and 40cm in diameter.
The head of the bomb disposal team Alexander Sotty said: “This is an English bomb. We must dig a hole before defusing the firing system to avoid any risk of an accident.
“If the worker had accidentally kicked the detonator, it could have caused 880m worth of damage.”
As well as evacuating the local residents the discovery also disrupted rail services to the Paris-Rouen-Le Havre line as well as causing severe delays on the nearby
A13.
Dozens of police were called to the scene who established a security perimeter.
Mr Sotty said: “A security perimeter of 340 meters around the site has been established.
“During the defusing of the bomb, nearly 3,000 people were evacuated as a precaution.
“Four gyms and a multipurpose room in Mantes-la-Jolie and Limay have been made available to accommodate them.
"We made immediate steps to accommodate residents and get warm.”
A total of seven bomb disposal experts worked on the device with the operation starting just before 10.30pm.
Police have since given the all-clear and residents were allowed back into their homes.