A practice bomb has gone missing from an aircraft on a routine training flight in Lincolnshire.
Ministry of Defence (MoD) officials believe the bomb was lost by a Harrier from RAF Wittering flying to the Holbeach drop zone.
A ground search of the flight path was ongoing on Tuesday night and police have been informed.
Anyone finding the light blue practice bomb with brown tape around the bottom of it should contact police.
The RAF said it was "highly likely" it had landed harmlessly in the sea or on land within the drop zone, but the possibility of it falling into land outside the target area could not be discounted.
An RAF spokesman said: "We are very aware of the risks of flying and safety remains one of our highest priorities.
"An investigation has been convened to determine the cause of the bomb's release.
"A small amount of explosive is contained in the device - roughly in comparison with a commercial firework."