A Trident nuclear missile failed, plopping into the Atlantic during a test launch attended by the British defence secretary last month, raising questions about the efficiency and effectiveness of Britain’s nuclear deterrent.

The Ministry of Defence said that an “anomaly occurred” during the exercise involving the submarine HMS Vanguard on 30 January that was “event specific”, implying it would not have happened in the unlikely event of a war.

No further details about the embarrassing failure were provided, although the MoD insisted that “there are no implications for the reliability of the wider Trident missile systems and stockpile”. Defence sources added that Grant Shapps was present at the time the test took place.