In a statement, Defence Secretary Grant Shapps confirmed "an anomaly did occur" when a rare operation was held on HMS Vanguard on 30 January - but stressed this was "event specific".

"Nor are there any implications for our ability to fire our nuclear weapons, should the circumstances arise in which we need to do so," Mr Shapps said.

His statement added: "The Trident missile system remains the most reliable weapons system in the world, having successfully completed more than 190 tests."

Mr Shapps said the government had "absolute confidence" in the UK's nuclear deterrent - and that there were "no implications for the reliability of the wider Trident missile systems and stockpiles".

He went on to warn: "The UK's resolve and capability to use its nuclear weapons, should we ever need to do so, remains beyond doubt."

The fault had something to do with it being a test-firing, with a source saying that the launch would have been successful had it been carried out for real with a nuclear warhead.

The Sun newspaper first revealed the drama, saying Mr Shapps had been onboard the submerged submarine at the time.

It is the second Trident missile failure in a row for the Royal Navy's ageing nuclear weapons fleet after a problem with another test-firing in 2016, when a missile flew in the wrong direction.

The UK has four nuclear-armed submarines. The country's nuclear deterrent requires at least one of them to be continuously at sea to deter nuclear threats from enemies such as Russia and to be ready to respond should the worst happen and the UK or its allies face a nuclear attack.

Mr Shapps said a routine "Demonstration and Shakedown Operation" had been held, which is when a ballistic missile submarine completes scheduled deep maintenance.

A series of weapons and sub-system tests are performed to assess the submarine and crew - and this culminates in the firing of an unarmed Trident II D5 missile.

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While details surrounding submarine operations are not normally disclosed, Mr Shapps said his statement was in recognition "of the level of interest" in what happened, all while protecting national security.

He added there was continued investment in the next generation of ballistic submarines, extending the lifespan of Trident missiles and replacing warheads.

"Sustaining the deterrent and renewing it for the future is a truly national endeavour," Mr Shapps said - describing it as "the ultimate security insurance policy".

On 30 January, first-stage boosters did not ignite and the 60-tonne missile - fitted with dummy warheads - splashed into the Atlantic Ocean and sank.

A source told The Sun: "It left the submarine but it just went plop, right next to them."

A search was immediately initiated to recover the highly sensitive munition.

HMS Vanguard, which has just completed a £500m overhaul, was undergoing a final round of tests before it returns to nuclear patrols.