One Royal Navy personnel has been killed and another is in a life-threatening condition after a shooting on board nuclear submarine HMS Astute.
A third Royal Navy serviceman was arrested after police were called to Southampton docks where the vessel has been berthed since Wednesday.
The BBC's Jonathan Beale understands that a crew member shot two of his crew mates before being overpowered.
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A police spokeswoman said the incident was not linked to terrorism.
'Matter of urgency'
She said there was no public safety risk and the area was sealed off.
BBC News understands the arrested man was handed over to Hampshire police by Ministry of Defence police.
An MOD spokesman said: "Two Royal Navy personnel have been involved in a firearms incident at Southampton docks where HMS Astute is alongside.
"Sadly, one has now died as a result of his injuries. The Royal Navy is now attempting to inform their families as a matter of urgency.
"A third Royal Navy serviceman has been arrested by Hampshire Constabulary and is now in custody."
Defence Secretary Liam Fox said: "I am greatly saddened to hear of this incident and of the death of a Royal Navy service person in this tragic incident.
Largest submarine
"It is right and proper that a full police investigation is carried out and allowed to take its course. My thoughts and sympathies are with those who have been affected and their families."
A spokesman for Southampton City Council confirmed their Mayor Carol Cunio, chief executive Alistair Neill and leader Royston Smith were all on board at the time of the shooting but were not injured in the incident.
They were not available for comment.
Brian Cedar, who lives in Hythe marina, said: "I saw at least six people carry a stretcher off the gangway into a waiting ambulance.
"There were a couple of forensic people who have now left.
"If you can have a shooting like this on a nuclear submarine it is worrying."
A South Central Ambulance spokeswoman said the dead man has passed away at the scene.
All concerned family members are asked to call the MOD's Joint Casualty and Compassionate Centre on 0845 7800 900.
