He visited HMS Victorious, one of the four Trident-carrying subs (Picture: MOD).
The Prime Minister has stepped aboard one of Britain's Vanguard-class submarines for the first time.
Boris Johnson and new Defence Secretary Ben Wallace were given a tour of the Clyde Naval Base near Glasgow, more commonly known as Faslane.
It is the Royal Navy's main presence north of the border, and home to the UK's nuclear deterrent.
Stepping on board HMS Victorious, Mr Johnson expressed surprise at the size of the submarine. He said it was "like a floating town".
Victorious is a Vanguard-class submarine, due to be replaced by the Dreadnought-class, still under construction and due to be completed around 2028.
Watch: will defence be prioritised under Boris Johnson?
Professor Malcolm Chalmers, Deputy Director-General of the Royal United Services Institute (RUSI), told Forces News ‘the jury is still out’ on how defence will fare under Mr Johnson’s leadership: “Jeremy Hunt made a very generous offer in terms of funding for the Ministry of Defence but Boris Johnson stuck to existing Government policy.
“The Prime Minister’s large spending commitments have been on tax-cuts, police funding and potentially social care.
Defence hasn’t figured on that list.”
Professor Chalmers speculated that ‘capable’ defence ministers Penny Mordaunt and Tobias Ellwood were dropped for their lack of support for Johnson in the Tory leadership race, but remained confident in the influence of the latest defence appointments.
“The new ministers are all people who are very strongly committed to defence and security.
“They’re going to be a strong, internal lobby… within the Government for defence to get its due share”
One of Boris Johnson's first tasks as Prime Minister was to write the letters of last resort.
Kept in the safes of Britain’s nuclear-armed submarines, they are instructions to the boat's captains are only to be opened and acted on in the event of a military strike that leaves the UK with no government.