The Luftwaffe 500-lbs SC250 bomb was discovered by a dredger preparing the channel for the arrival of Royal Navy's new 65,000-tonne aircraft carriers.
Lieutenant Mike St Pierre, the naval officer leading the bomb disposal party, said: "Despite being old, these devices can pose a very serious threat."
Hampshire Constabulary said it had "activated a pre-arranged multi-agency plan" with Ministry of Defence Police, the Royal Navy, Portsmouth City Council and others.
All ferries were stopped to and from Portsmouth, Britain's only island city, and trains between Portsmouth and Southsea station and Portsmouth Harbour were suspended.
Lieutenant Mike St Pierre, the naval officer leading the bomb disposal party, said: "Despite being old, these devices can pose a very serious threat."
Hampshire Constabulary said it had "activated a pre-arranged multi-agency plan" with Ministry of Defence Police, the Royal Navy, Portsmouth City Council and others.
All ferries were stopped to and from Portsmouth, Britain's only island city, and trains between Portsmouth and Southsea station and Portsmouth Harbour were suspended.