Ministers have been accused of covering up a failed test of the Trident nuclear deterrent weeks before a crucial Commons vote on the future of the submarine-launched missile system.
Labour former defence minister Kevan Jones demanded an inquiry into Sunday Times claims that the launch of an unarmed Trident II D5 missile from a British submarine off the coast of Florida in June malfunctioned.
The newspaper said the cause of the failure remained top secret but questions could be asked over the Government's failure to publicise the failed test weeks before MPs approved the £40 billion Trident renewal programme in July.
Previous tests have been publicised by the Government.
Mr Jones told the newspaper: "The UK's independent nuclear deterrent is a vital cornerstone for the nation's defence.
"If there are problems, they should not have been covered up in this ham-fisted way.
"Ministers should come clean if there are problems and there should be an urgent inquiry into what happened."
A Government spokesman said: "The capability and effectiveness of the Trident missile, should we ever need to employ it, is unquestionable.
"In June the Royal Navy conducted a routine unarmed Trident missile test launch from HMS Vengeance, as part of an operation which is designed to certify the submarine and its crew.
"Vengeance and her crew were successfully tested and certified, allowing Vengeance to return into service. We have absolute confidence in our independent nuclear deterrent.
"We do not provide further details on submarine operations for obvious national security reasons."