The Royal Navy bomb squad from Plymouth will detonate more than 100 out of date flares at Deep Point this evening.
Isles of Scilly Sergeant Darran White said islanders and visitors should not be concerned if they hear a "very loud bang", which should be audible across St Mary's between 7pm and 8pm. He asked that people avoid the area during the hour.
The move follows an out of date flare being handed in to police. As Scilly has no system for disposing of them, officers called the Explosive Ordnance Disposal (EOD) team in Plymouth, who arrived on the island today.
Police decided to hold an amnesty and deal with any unsafe devices at the same time, resulting in "well over 100" expired flares being handed in within 24 hours.
Sgt White told This is Scilly: "The bomb disposal team will be doing a controlled explosion of all the old flares, which just means we don't have to worry about anyone fooling around with unsafe flares and accidentally setting them off in someone's home or elsewhere. Because they are maritime distress flares they would provoke a full response from the coastguard and search and rescue, which we're trying to avoid."
The EOD team will dig a deep hole on the beach, pack all the flares inside and place explosives on top, aiming the charge downwards. The incoming tide with then wash the hole away.