The boy found the rusty device on the beach opposite Fort Henry, Grouville, on Tuesday, and took it home as a souvenir.
It was only when a friend suggested that the mortar could be dangerous that the police were called.
Bomb disposal officer Stuart Elliott arrived at the address near Grouville Church that evening.
‘It turned out to be a five-centimetre German mortar bomb.
It was live, and the explosives were in as good a condition as when it was made,’ he said.
‘It would have done a fair bit of damage if it had gone off and could have caused serious injury or even death.
‘He was very fortunate.
Any movement could have caused problems.’ The mortar was about eight inches long, with a fin at one end and an aluminium fuse at the other.
It was destroyed at a secure location in the north of the Island.
A States police spokesman today urged anyone who finds any unexploded munitions not to touch them and to contact them immediately.
