The alert at RAF Fairford in Gloucestershire lasted for several hours but the base was later declared safe.

The missile was being loaded onto a B1 American aircraft on Thursday morning, but a spokesman confirmed it was not a Cruise missile.

Several nearby roads were closed off until it was certain the area had been made safe by service personnel.

The bomb has been thoroughly examined and is not damaged, said a US airforce spokesman.

A full investigation is now under way to work out exactly how the mishap occurred.

B1s have successfully carried out raids on Serbian targets

Two B1 bombers from Fairford were involved in overnight raids on the Serbian capital Belgrade, in Nato's third week of attacks on Serb targets.

Fuel depots in southern and northern Serbia were also attacked, according to Serb media.