King Charles has stepped on board a Royal Navy nuclear submarine just days after Vladimir Putin's Russia threatened to resume testing nukes at a former Soviet facility in the Arctic.

His Majesty warmly greeted dozens of serving submariners at the Royal Naval Armaments Depot (RNAD) at Coulport, part of HM Naval Base Clyde, in Faslane, Scotland.

The King was welcomed on board one of the Vanguard-class submarines which acts as Britain's nuclear deterrent and he met with Gold Deterrent Pin holders.

According to the British Forces Broadcasting Service (BFBS) holders of the presitigous pin "are recognised as service personnel who have spent the most time under the water" serving the nation.

Submariners stationed on nuclear subs can spends months at sea in total secrecy as they act of the nation's first line for defence in the doomsday scenario of atomic war.

In 1974 the then-Prince Charles qualified as a helicopter pilot and flew Wessex aircraft as part of the 845 Naval Air Squadron from the commando aircraft carrier HMS Hermes.

The King, who has been undergoing treatment for cancer in recent months, wore full Navy uniform for his visit to the base on Thursday where he was welcomed by First Sea Lord Admiral Sir Ben Key KCB CBE and received the royal salute from several senior members of the Navy.

It marks the first time the King has visited the base as head of the armed forces, though he did visit in 2011 and 2013 as the Prince of Wales.

His visit this week comes after Russia announced on Tuesday it was "fully ready" to resume live nuclear testing at a former Soviet Arctic facility on the Novaya Zelmlya archipelago.

Russian Rear Admiral Andrei Sinitsyn has said the nuclear testing facilities, which were closed in 1990, are now once again fully operational.

He said: "The test site is ready to resume full-scale testing activities. The laboratory and testing facilities are ready. The personnel are ready."

Speaking about the King's visit to Faslane, Commodore Paul Dunn OBE, of the Submarine Service, said: "His Majesty's visit, his first here as commander-in-chief of the British armed forces, acknowledges the valuable service of our submariners.

"The Royal Navy's unbroken 55-year record of continuous at sea deterrence is a tremendous source of pride, showcasing the skill, professionalism and dedication of our defence enterprise.

"This remarkable national endeavour, entrusted to us since 1969, has seen our submariners and their families make significant sacrifices in the defence of our country."

Chief Petty Officer (CPO) Longhurst and his family were among the submariners who met the King. He said: "Meeting the King with my wife and shipmates today was a truly special moment, marking the culmination of my service as a Royal Navy submariner.

"It has been a challenging but fulfilling 23 years, seven of which have been underwater.

"His Majesty's visit today not only highlights the importance of the job we do in keeping the nation safe, but it also emphasises the invaluable support of our loved ones who stand by us through every deployment.

"Our families are the unsung heroes and today recognises that at the highest level."

The geographic position of a Vanguard-class submarine on patrol is a secret not shared with the majority of the crew. While at sea, submariners have little contact with the outside world.

However, one of the most important links with home is the arrival of the "Family Gram", a 120-word one-way message that arrives weekly.

Patrolling submariners will also receive a snapshot of local and international news stories, as well as sporting results - all of which are vetted by the commanding officer.