GAVIN Williamson has paid tribute to the submariners who have worked on the UK's "longest sustained military operation" as the House of Commons marked 50 years of the country's continuous at-sea nuclear deterrent.

And the Defence Secretary told MPs there would be a new service pin for those who had served 10 patrols as part of Operation Relentless.

But the SNP lashed out at Britain’s nuclear arsenal with its defence spokesman Stewart McDonald branding Britain’s nuclear deterrent a "big-willy gesture of a small-willy nation".

Opening a Commons debate, Mr Williamson said "half a century ago HMS Resolution glided into the Clyde and sailed into the history books" as the first submarine to take part in the mission.

He said since then "thousands of submariners have followed in the wake of Resolution's crew, conducting vital work, unseen and undetected, every minute of every day".

The Secretary of State said that while those who served on them were already honoured with the deterrent patrol pin known as the "bomber pin", the new "silver bomber pin" would be handed out, made from metal taken from the now-decommissioned HMS Resolution.

Addressing those who no longer believed the UK should have a continuous at-sea presence, Mr Williamson argued that those who believed it was “simply a Cold War relic" should be reminded that the "the nuclear dangers have not gone away", saying the "geopolitical situation is more unstable than ever before".

His Tory colleague Simon Clarke, who represents Middlesbrough South and East Cleveland, agreed, saying he saw "no way in which these weapons can be uninvented and, therefore, I can see no realistic situation in which we will ever be able to completely disarm".

Mr McDonald went on: "Only in this House of Commons, at this time against the backdrop of a major constitutional crisis where each day is worse than the last, could it be thought of as a good use of time to back-slap each other on the UK being 50 years as a marine nuclear power.

"Anyone who thinks that is a good use of our time right now, frankly, is off their head."

The Glasgow MP claimed it was "no surprise" MPs wanted to "hark on these symbols of power" at a time when Theresa May had "the begging bowl" out in the EU.

He added that there was no military or economic case for a continuous at-sea deterrent and said the UK had become an "irresponsible" nuclear power.

In contrast, Labour backbencher Madeleine Moon, who is President of the Nato Parliamentary Assembly, replied: "I want to say on behalf of the Nato alliance that that alliance greatly values the UK deterrence and would actually be grateful for the common sense, trust, and belief in the UK's deterrence and our capacity and willingness to dedicate to their stability and security.

"They would actually be horrified by Mr McDonald's comments."

Her Labour colleague Nia Griffith, the Shadow Defence Secretary, said Labour was committed to the renewal of Britain's nuclear submarines but stressed how using a nuclear weapon would be a "catastrophic failure".

She said: "The threats facing the UK are real and undiminished. There is a need to deter against the use of nuclear weapons in all circumstances. None of us ever wants to be in a position where the deterrent is used.

"If we ever got to that situation, it would represent a catastrophic failure of our rules-based system and the very concept of deterrence."

Senior Tory MP Julian Lewis, who chairs the Commons Defence Committee, paid tribute to submarine crews, calling them the "silent guardians" of the country.

The Hampshire MP insisted the nuclear deterrent was necessary to prevent other nuclear powers targeting the UK, believing there would be no repercussions.