A Trident missile has misfired and crashed into the ocean off the coast of Florida during a rare test launch by a British nuclear submarine in an embarrassing blow for the Royal Navy.

The Ministry of Defence on Tuesday night confirmed an “anomaly” had occurred with the drill involving HMS Vanguard, but a spokesperson insisted that the nuclear deterrent – the cornerstone of the UK‘s defences – “remains safe, secure and effective”.

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, which happened on 30 January, saying that Defence Secretary Grant 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.

The UK has four nuclear-armed submarines that are charged with ensuring one boat is 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.

Outlining what happened, The Sun said the Trident 2 missile was propelled successfully from under the water into the air by compressed gas in the launch tube.

But its 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 newspaper: “It left the submarine but it just went plop, right next to them.”

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

Indicating the gravity of the event, a written ministerial statement is due to be released to parliament at around midday on Wednesday.

Rishi Sunak will also likely be asked about what happened when he faces Prime Minister’s Questions.

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

The Ministry of Defence spokesperson said, despite the glitch, the submarine and her crew “have been proven fully capable of operating the UK’s Continuous At-Sea Deterrent, passing all tests during a recent demonstration and shakedown operation (DASO) – a routine test to confirm that the submarine can return to service following deep maintenance work”.

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The spokesperson said: “The test has reaffirmed the effectiveness of the UK’s nuclear deterrent, in which we have absolute confidence.

“During the test an anomaly occurred.

“As a matter of national security, we cannot provide further information on this, however we are confident that the anomaly was event specific, and therefore there are no implications for the reliability of the wider Trident missile systems and stockpile.”

Ahead of the launch last month, the US National Geospatial-Intelligence Agency had issued a warning to shipping that plotted the missile’s expected course to an impact in the mid-Atlantic.

The “hazardous operations” warning said that the missile was expected to travel some 3,700 miles before crashing into the sea between Brazil and west Africa.

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The failed launch came eight years after the Royal Navy last test-fired an unarmed Trident II D5 ballistic missile.

Back in 2016, the missile was launched from HMS Vengeance and flew in the wrong direction.

Defence sources told The Guardian at the time that it did not veer off in the wrong direction because it was faulty but because the information relayed to it was incorrect.

The Trident system has completed more than 190 successful tests during its time in service.