The first steel for the vessel Warspite has been cut at BAE Systems' shipyard in Barrow-in- Furness, Cumbria.
It is the third of four new Dreadnought class submarines built to carry Trident ballistic missiles at an estimated cost of 31 m.
He said the milestone was a significant step forward in a programme that was supporting thousands of jobs and apprenticeships across the country.
He added that it would be protecting the UK and our allies for decades to come.
The programme is one of the most complex engineering projects undertaken in the UK, according to the government.
In the early 2030s, the four new nuclear submarines - Dreadnought, Valiant, Warspite and King George VI - will replace the Vanguard Class fleet.
On Warspite, work will continue, along with the first two boats Dreadnought and Valiant.
Each is approximately 500 ft 153.6 m long and contains 42 km 26.4 miles of pipework and more than 20,000 cables.
Rear Admiral Donald Doull, Dreadnought's senior responsible officer, said that the steel cutting was an important step in the ongoing delivery of the continuous at sea deterrent. Successfully constructing the new fleet was a challenge that will take the determined effort of everyone with a responsibility for supporting the programme.
BAE Submarines' managing director Steve Timms called the steel cutting a really significant moment for the thousands of employees at BAE Systems and across the submarines enterprise.