A British nuclear missile suffered an apparent misfire during testing, raising concerns about the effectiveness of the UK's deterrent.
The Ministry of Defence confirmed that an "anomaly" occurred on one of Britain's Trident nuclear-armed submarines on January 30.
Defence Secretary Grant Shapps, who was on board HMS Vanguard at the time, said Britain's nuclear capability "remains beyond doubt".
First Sea Lord Admiral Sir Ben Key was also present as HMS Vanguard was tested after a refit lasting more than seven years.
It is the second such incident in recent years after a missile fired by HMS Vengeance was reported to have malfunctioned in 2016.
One of Britain's four nuclear-armed submarines is on patrol at all times in a policy known as continuous at-sea deterrence.
The UK is one of three Nato countries, with the US and France, that has nuclear weapons, which the alliance regards as its ultimate deterrent against potential aggressors such as Russia.
The Labour opposition called for Mr Shapps to "reassure parliament that this test has no impact on the effectiveness of the UK’s deterrent operations".
"Reports of a Trident test failure are concerning," shadow defence secretary John Healey said.
The firing of a Trident missile was part of the submarine's "demonstration and shakedown" testing after its lengthy maintenance.
Mr Shapps said the submarine and crew "were successfully certified and will rejoin the operational cycle as planned".
"On this occasion, an anomaly did occur, but it was event specific and there are no implications for the reliability of the wider Trident missile systems and stockpiles," he said.
"Nor are there any implications for our ability to fire our nuclear weapons, should the circumstances arise in which we need to do so."
The Ministry of Defence said it could not provide further details. A report in The Sun said the misfire occurred during an exercise the coast of Florida.
It said a dummy Trident missile was launched but its boosters did not ignite and it "just went plop, right next to them", according to an anonymous source.
The UK's nuclear-armed submarines carry American-built Trident 2 D5 nuclear missiles, which can fire at targets more than 6,000 kilometres away, according to the Royal Navy.
Each Vanguard-class submarine can hold up to 16 intercontinental ballistic missiles, but will only carry up to eight Trident rockets and up to 40 nuclear warheads.
The V-class is due to be replaced by the bigger Dreadnought-class submarines in the 2030s.
Between £31 billion ($39.07 billion) and £41 billion has been set aside for the wider programme of replacing the Vanguard-class submarines, which was approved in 2016.
