Camilla, Duchess of Cornwall, launched HMS Astute, the Royal Navy's new nuclear submarine, at the shipyard where the vessel was built in Barrow, Cumbria.
Crowds of schoolchildren, navy personnel and local people lined the streets to welcome the duchess as she prepared to launch her first ever vessel.
A Royal Navy Merlin helicopter performed the warm-up for spectators as they waited for the launch to begin.
The duchess was joined by the Bishop of Carlisle, the Right Reverend Graham Dow, who blessed Astute before her launch.
A dummy Cruise missile inside Astute gives a hint of the nuclear vessel's firepower, which will be greater than that of any previous British submarine.
Instead of champagne, the duchess launched Astute with a bottle of beer specially brewed by the submarine's own crew, in line with tradition at the Barrow shipyard.
Dignitaries gathered to watch as the £3.5bn submarine - which can create her own oxygen and fresh water - was finally launched.
Wheeled out on a giant mechanical trolley, Astute took three hours to leave the hall and was not due to be lowered into water until the following day.