A £1.2bn contract has been agreed to build a new submarine for the Royal Navy.
The deal, awarded by the Ministry of Defence (MoD) to BAE Systems, will safeguard 3,000 jobs at the company's Barrow shipyard in Cumbria.
The submarine, to be called HMS Audacious, will be the fourth of seven Astute Class boats being built for the Navy. It will join Astute, Ambush and Artful in the growing fleet.
The first two submarines, Astute and Ambush, are currently undergoing sea trials to test their systems ahead of full service. These trials assess their ability to dive to deep depths and fire missiles.
A further £1.5bn has also been committed to three submarines yet to be built, which will complete the fleet. It will allow vital preliminary work to start.
Commenting on the announcement, Rear Admiral Simon Lister, the MoD's director of submarines, said: "The Astute Class will become the jewel in the crown of the Royal Navy's Submarine Service and boasts much greater firepower and more advanced sonar and communications than ever before.
"These submarines represent a huge leap forward in technology and will operate all over the world with the Royal Navy.
"These boats provide the optimum capability a submarine can offer in land strike, strategic intelligence gathering, anti-submarine and surface ship warfare, and protection of the strategic deterrent."
The Astute class submarines are powered by nuclear energy which means they never need to refuel. In theory they can stay underwater forever, only re-surfacing to take on supplies for the crew.
They are fitted with the most advanced sonar systems available and are quieter than older submarines. The sonar system has the processing power of 2,000 laptops and can spot and track ships 3,000 miles away.
At around 320ft (97m) from bow to stern they are about 50% bigger than the Royal Navy's current Trafalgar Class submarines. They carry on board a mix of Spearfish torpedoes and Tomahawk land-attack missiles.
The submarines will also make their own oxygen from seawater.
The money is coming from a pre-allocated budget. In the Autumn Statement the Chancellor said the MoD could have more time to spend about £1bn that it has yet to use from this year's budget.
It was thought that the Treasury might request the money be returned, but George Osborne has allowed the department a period of flexibility.