Parts of Holkham beach were cordoned off after a metal detector found a grenade.
The coastguard and police were called to the beach on Thursday afternoon last week after an “orange rock” was found by a man with a metal detector.
Pictures shown to explosive experts revealed that this was a Second World War phosphorus grenade.
Adam Turner, from the Humber coastguard said: “They said it was inert and nothing to worry about.
“It was left there for the tide to take away.”