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Theo and Benoit Donaldson, 13, found the device using a metal detector in a park in Leytonstone, east London.

They dug down 1ft to recover the WWII Mills bomb and showed it to mum Marie, 46, who told them to take it home so they could look it up online.

On the way they threw it between each other and even dropped it several times.

Only when Marie’s policeman brother saw a photo of the grenade she had emailed him did they realise the danger.

He alerted the bomb squad who removed and destroyed it.

Marie said: “It was a big bang. That’s when the shock really set in.”