The 500 pound World War Two device was picked up during dredging work.
£300m Cocaine Haul Seized In Southampton
A record £300 million haul of cocaine has been seized and seven alleged members of an international drugs gang have been arrested.
It is the biggest haul of Class A drugs ever found in the UK, officials from the UK Border Agency (UKBA) and the Serious Organised Crime Agency (Soca) said.
French authorities were alerted to the suspicious £1 million craft Louise while it was in the Caribbean in May and it was then tracked to Southampton, on its way to Holland.
Officials spent six days searching the vessel and found the drugs packed in a specially-designed compartment beneath the boat's bathing platform.
It is understood the cocaine, which originated in South America, was packed inside the boat while it was in Venezuela.
The drug was 90% pure, while the average purity of cocaine seized at the UK border is just 63%, officials said.
The haul is estimated to be worth between £50 million wholesale and £300 million on the streets.
Since the drugs were found in June, the UKBA has helped Dutch police track members of the gang and seven men were arrested during early-morning raids yesterday - two 44-year-olds in Amsterdam, a 60-year-old in Meppel, two, aged 32 and 34, in Heusden, and a 27-year-old in Waalwijk.
A total of 100,000 euros (£87,300), two Harley Davidson motorcycles, two firearms, a silencer and some ecstasy were also seized.
Immigration Minister Damian Green said:
''This was a significant drugs seizure which was made possible by the co-operation of our international partners.
''UK Border Agency staff have shown vigilance, dedication and determination to uncover this shipment.
''Our efforts have helped bring an international crime gang to book and the message is clear: we will investigate and prosecute anyone who tries to smuggle banned substances through UK borders.
''By keeping the border secure we play a key role in stopping drugs entering the UK and in reducing the harm they do to our communities.
''We aim to protect society from the violence and corruption that always accompanied the trade in illegal goods.''
The Dutch police were acting on intelligence provided by Soca's international network and the French Customs Investigation Service (DNRED).
UK Border Agency officials called the hiding of a record drugs haul in the 44-tonne, £1 million luxury yacht ''ingenious''.
''The cocaine took up a space of about four cubic metres and was fitted neatly under the diving platform aft with access from the engine room. It required some really good blanking panels to hide them and it was ingenious,'' a spokesman said.
The hiding place was so good it took specially trained officers six days to find the haul.
''The concealment was a professional job. It's a case of being methodical to find it,'' said Doug McLellan, the head of maritime operations at the UKBA.
''Six days might sound like a long time but there is a lot of areas on a yacht of this size, so it's a painstaking process using trained officers, firstly looking through the obvious places and when those have been exhausted looking at the fabric of the boat.''
The yacht had been transported from the British Virgin Islands on board a specially designed Dutch-registered ship, Snoekgracht, owned by Seven Stars Yacht Transport along with 30 others. It was coming to Europe to be refitted.
The transport company was completely unaware of the drugs, the spokesman said.
Now the yacht, which was owned by those arrested, is at berth in Southampton docks and is Crown property.
640 men were killed when the South African troop carrier was hit by a cargo ship in 1917.
The 79-year-old man suffered a serious head injury in the Cranmer Road car park in Winton.
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