A Trident missile misfired and crashed into the ocean off the coast of Florida during a rare test launch.

The Ministry of Defence (MoD) confirmed an "anomaly" had occurred with during an exercise involving the nuclear submarine HMS Vanguard.

But a spokesperson insisted that the nuclear deterrent - the cornerstone of the UK's defences - "remains safe, secure and effective".

The fault, which was first reported by the Sun, is believed to have involved a dummy Trident 2 missile which was propelled into the air by compressed gas in its launch tube, but its so-called first-stage boosters did not ignite on January 30.

Officials said they could not say any more because the incident relates to national security, but they said there remained “absolute confidence” in Britain’s continuous at-sea nuclear deterrent.

HMS Vanguard is one of the four submarines, all of them based at HM Naval Base Clyde, that carry the American-built Trident 2 D5 nuclear missiles, the mainstay of Britain’s strategic nuclear deterrent.

An anonymous source quoted by the newspaper said: “It left the submarine but it just went plop, right next to them.”

The MoD said the “anomaly” during the exercise was “event specific”.

It is said to be the second misfiring in a row, with a test launch of a Trident missile by the Royal Navy off the coast of the United States in June 2016 also reported to have been a failure.

Labour's shadow defence secretary John Healey said: “Reports of a Trident test failure are concerning.

“The defence secretary will want to reassure parliament that this test has no impact on the effectiveness of the UK’s deterrent operations.”

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

A written ministerial statement is due to be released to parliament at around midday on Wednesday.

It's thought Rishi Sunak will also be asked about what happened during Prime Minister's Questions in the House of Commons.

Defence secretary Grant Shapps was on-board the 150-metre vessel at the time of the incident, a spokesman for the Defence Secretary confirmed.

First Sea Lord Admiral Sir Ben Key was also present at the time to mark what was the final exercise for Vanguard and its crew after undergoing a refit that took more than seven years, an MoD spokesman said.

The incident comes at a time of high global tension, with a war raging in the Middle East and Russia’s invasion of Ukraine approaching its two-year anniversary.

A spokeswoman for the MoD said: “HMS Vanguard 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.

“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.

“The UK’s nuclear deterrent remains safe, secure and effective.”