On Monday, Charles visited the BAE Systems shipyard in Barrow-in-Furness, Cumbria, to read the commissioning warrant at a formal ceremony to mark HMS Agamemnon entering operational service.
Nicknamed "Awesome Aggie", the vessel - named after the commander of the mythical Ancient Greek forces in the Trojan war - weighs 7400 tonnes and measures 97 metres long, as long as four buses.
Agamemnon will protect the UK's strategic nuclear deterrent, aircraft carrier strike groups and critical undersea infrastructure.
The vessel can strike land targets up to 1609 km away, serve as a launch pad for raids by teams of Royal Marines Commandos, and gather intelligence through state-of-the-art sensors.
More tests on site are expected to follow before it leaves the town for trials at sea, before going into active service.
HMS Agamemnon is the sixth of seven Astute-class Fleet submarines built for the Royal Navy.
Monday's ceremony also recognised those who built, staff and support it as Charles spoke with BAE employees involved in its construction as well as the boat's serving personnel and their families.
Dignitaries attending included Defence Secretary John Healey and Australia's High Commissioner to the UK, Stephen Smith.
"This is truly a remarkable manufacturing feat to take a piece of steel and to forge it into one of the most technologically advanced machines anywhere in the world," Mr Healey told those gathered.
Earlier, Mr Healey conducted a steel-cutting ceremony at BAE for King George VI, the fourth and final Dreadnought-class submarine.
The Dreadnought submarines, due to enter service from the early 2030s, are the replacement for the Vanguard-class submarines, which currently deliver the continuous at-sea deterrent for the Royal Navy.
Charles later visited Barrow Town Hall to bestow royal port status on the town in honour of its 120-year history as the home of UK submarine construction.
Britain and Australia have teamed up with the US on an AUKUS nuclear submarine agreement, which is under review by the Trump administration.
AUKUS, which was formed in 2021 to address concerns about China's rising military ambition, is designed to enable Australia to acquire nuclear-powered attack submarines in the 2040s.
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