Coastguards have reassured the public that an unexploded device found off the Purbeck coast but later lost track of is not a danger to shipping.

They were called at lunchtime on Saturday after a survey boat found the unexploded device about nine miles south of Lulworth.

However, the boat that found it did not mark the device and when the Navy’s Explosive Ordnance Disposal team was despatched to the scene it was unable to locate it again.

Ros Evans, watch manager at Portland Coastguard, said: “It was absolutely impossible to find it again. If it’s not marked then there’s not too much we can do about it.

“It’s in 35 metres of water and is not a danger to anything on the surface. There must be lots of it out there with the firing ranges.”

The Lulworth Ranges are part of the Armoured Fighting Vehicles Gunnery School and include the coast west of Kimmeridge Bay and east of Lulworth Cove.

There is a sea danger area that extends six nautical miles offshore between Lulworth Cove and St Aldhelm’s Head.

Vessels are asked to stay out of the zone during firing times.

Coastguards were also called to Mupe Bay, near Lulworth Cove, earlier on Saturday following reports of a missing diver.

A yacht reported the man missing as he was on his own, but he was soon located in shallow water.