The Royal Navy has issued a statement to say a 500m cordon is in place around the device, which is preventing cross-channel ferries from entering and leaving the ferry port.
The device, dating from the Second World War, was found by a dredging barge carrying out work in the harbour before the arrival of HMS Queen Elizabeth - the navy’s new 65,000-tonne aircraft carrier - into the naval base next spring.
The statement adds: ‘Victory Gate is closed to pedestrians. Naval base employees are instead having to enter through Marlborough Gate on Queen Street.
‘A plan is in place to tow the bomb out of the harbour where it will be destroyed in open waters by RN bomb disposal experts.’
Both Wightlink and Gosport ferry services are unaffected.
It’s the third bomb found in the harbour since September, the first being a British torpedo on September 10 which also resulted in a 500m exclusion zone being put in place. A second 500kg bomb was then blown up in the harbour later in the month.
A spokesman at Portsmouth Historic Dockyard said it would be closed to visitors until at least midday.