The first two boats, HMS Astute and Ambush, are currently undergoing sea trials.

BBC: HMS Audacious: MoD announces ?1.2bn submarine contract

Lt Cdr Ian Molyneux was killed by Able Seaman Ryan Donovan while HMS Astute was docked in Southampton in 2011.

BBC: Manchester

Rear Adm Corder said he was confident the HMS Astute incident was not "indicative of wider failings" within the submarine service.

BBC: 'No dedicated plan' led to HMS Astute grounding

In October 2010, the Anglian Prince went to the aid of the nuclear-powered submarine HMS Astute after it grounded off Skye.

BBC: Three-month extension' for Scotland's coastguard tugs

Southampton City Council leader Royston Smith and chief executive Alistair Neill tackled Able Seaman Ryan Donovan on board HMS Astute in April 2011.

BBC: George Medals for men who tackled gunman Ryan Donovan

Lt Cdr Ian Molyneux, 36, of Wigan, Greater Manchester, was shot at close range on board HMS Astute when it was docked in Southampton in April 2011.

BBC: Ian Molyneux inquest: Intended target describes escape