A British nuclear submarine test saw a Trident missile "dramatically misfired" before it crashed into the ocean.
An "anomaly occurred" during an exercise on January 30 aboard the HMS Vanguard, The Ministry of Defence (MoD) confirmed.
Officials said they could not reveal any more information as the incident relates to national security, but insisted there remained "absolute confidence" in Britain's at-sea nuclear deterrent.
Sources claim the Trident 2 successfully "left the submarine" but "just went plop" into the ocean next to the submarine and sank.
An 'anomaly occurred' during an exercise on January 30 aboard the HMS VanguardGettyThe Trident 2 missile was propelled into the air by compressed gas in its launch tube, however its "first stage" boosters did not ignite, the Sun reports.
HMS Vanguard carried carried out the drill off the coast of Florida while Defence Secretary Grant Shapps was on-board the vessel.
The missile was expected to travel 3,500 miles and land somewhere between West Africa and Brazil in the Atlantic Ocean.
The incident is the second consecutive failed launch after a missile was unsuccessfully fired from HMS Vengeance during a test in 2016.
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An investigation into the failed trial has been ordered to determine the cause, while a search will be carried out to recover the Trident 2 from the ocean.
The missiles are designed to burst into the edge of space and track their position, before re-entering the atmosphere and diving back to earth to hit its target.
It is believed the £4billion submarine must successfully pass its test in order to re-enter service.
Labour has said the "concerning" reports about the test failure questions the effectiveness of Britain's nuclear deterrent.
Shadow defence secretary John Healey has called for assurances, saying: "Reports of a Trident test failure are concerning.
The HMS Vanguard submarine spent seven years undergoing a refit in Plymouth
Getty"The Defence Secretary will want to reassure Parliament that this test has no impact on the effectiveness of the UK's deterrent operations."
An MoD spokesperson 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."