Astute is due to go into full service in 2010
The Royal Navy's biggest and most powerful attack submarine has set sail from the Cumbrian shipyard where it was built for sea trials.
Astute set sail from Barrow for its first set of trials before heading to a new home at Faslane in Scotland.
Measuring nearly 328ft (100m) from bow to stern, Astute is armed with 38 torpedoes and missiles - more than any previous Royal Navy submarine.
The BAE Systems-built vessel is due to go into full service next year.
Astute will be able to circumnavigate the entire globe while submerged and advanced nuclear technology means that it will never need to be refuelled.
Defence Equipment and Support Minister Quentin Davies said: "This is a significant milestone for Astute as she sails for the first time.
"The Astute class of submarines will deliver a step change in capability for defence in terms of anti-submarine and anti-surface warfare, protecting the deterrent, and providing land attack and intelligence gathering.
"Astute will now begin a set of sea trials ahead of her full acceptance with the Royal Navy next year."
Rear Admiral Simon Lister oversaw the final stages of the submarine production in Cumbria and preparation for sea trials.
He said: "Building a nuclear submarine is a huge challenge and demands the highest standards of design, engineering and manufacturing to ensure she can safely perform her demanding duties.
"Astute represents the sum of thousands of individual efforts, and everyone involved in her creation can be proud."
The submarine is expected to arrive in Faslane on The Clyde, later this week.