The item was found by magnet fishers yesterday.
After the discovery the mortar was taken to Eye to be detonated - with residents hearing a loud bang just before 9.30pm.
A spokesman for Cambridgeshire police said: “Yesterday at the embankment two magnet fishers came across a suspected explosive device
“With the help of our friends at the Explosive Ordnance Disposal (EOD) we established the device was a spigot mortar, potentially from WW2.
“The device was secured and disposed of successfully in the Eye area, with the EOD officers even allowing the police to press the red button!
“If you heard a loud bang around 21:40 in this area then don’t worry, this is why.”
The police spokesman said the device was taken to a location in Eye ‘completely away from roads, pedestrians and any buildings.’