Shops were evacuated and trains and ferries cancelled after a 500lb bomb was discovered during dredging work for the Royal Navy's new aircraft carriers in Hampshire.
Residents were advised to stay in their homes following the discovery of the German Second World War bomb in Portsmouth Harbour.
Several pieces of unexploded ordnance have been found since work started to deepen the harbour to enable the next generation of carriers to access the Navy base.
A Navy spokesman said: “The German SC250 bomb was found in the early hours of this morning in the excavator head of a barge dredging the harbour – part of a raft of infrastructure upgrades taking place in readiness for the arrival of the Royal Navy's new 65,000-tonne aircraft carrier.
“Divers from the Royal Navy's Portsmouth-based Southern Diving Unit 2 are on the scene.”
Lieutenant Mike St Pierre, the officer leading the bomb disposal team, said: “At this stage it is too early to tell when normality will be restored to Portsmouth Harbour.
“But my team of Royal Navy divers are assessing the situation to work out how we can remove the bomb swiftly and safely with the help of the contractors, the Queen's Harbour Master, Hampshire Constabulary and our operations centre.
“As always, the Royal Navy stands ready to respond to these call-outs and keep people safe. Despite being old, these devices can pose a very serious threat.”
The SC250 bomb weighs 500lb and contains 290lb of high explosives.
Hampshire police tweeted: “We've activated a pre-arranged plan after #unexploded #ordnance found in #Portsmouth Harbour this morning (Feb 22) “Residents of Spice Island and Portsmouth Harbour, stay in your homes, open windows, move away from outer walls.”
