Britain and America gave contrasting accounts of a Trident missile failure following 24 hours of chaos over the incident which has seen Theresa May accused of a "cover-up".
There were immediate calls for an inquiry after a newspaper reported there had been a "serious malfunction" during an unarmed Trident test in June last year and that the missile may have "veered off in the wrong direction towards America".
In a statement the British Government said the Trident nuclear submarine had been "successfully tested and certified, allowing Vengeance to return into service".
Theresa May appeared on the BBC's Andrew Marr show and refused four times to say that she knew about the Trident misfire off the coast of Florida in the weeks before she forced a Commons vote calling for the renewal of the nuclear deterrent.
May said only that she has "absolute faith in our Trident missiles" and that "I think we should defend our country."
Today during a regular Downing Street briefing for the media, May's official spokeswoman admitted that the Prime Minister was informed about the test before she addressed MPs on the £40 billion renewal of the nuclear deterrent last northern summer.
The spokeswoman said: "The Defence Secretary [Sir Michael Fallon] and the Prime Minister are routinely informed when one of these specific 'demonstration and shakedown' operations are planned and on the outcome of them.
"In this instance, that was in June so it was under the then Prime Minister [David Cameron]. On taking office, the current Prime Minister was briefed on a range of nuclear issues, including this."
The spokeswoman declined to say whether May was informed of a malfunction in the missile system, stating it was not Government policy to discuss operational details of tests in public, and telling reporters that she did not anyway "accept the premise of the question".
"We have been clear that the submarine and the crew were successfully tested and certified," said the spokeswoman.
Fallon was summoned to the House of Commons to answer questions from backbenchers.
He repeatedly refused to discuss details of the launch. However, as he addressed the comments American broadcaster CNN began releasing details of the "failure", which it said had been briefed by US officials.
CNN reported an unnamed US defence official with direct knowledge of the incident had confirmed the unarmed Trident II D5 missile veered off course after being launched from a Royal Navy submarine off the coast of Florida.
The US official was reported to have said the altered trajectory was part of an automatic self-destruct sequence triggered when missile electronics detect an anomaly.
Mary Creagh, a Labour MP, said: "You have advised us not to believe everything we read in the Sunday newspapers. But should we believe the White House official who, while we've been sitting here debating, has confirmed to CNN that a missile did auto self-destruct off the coast of Florida?
"And if that is the case, why is the British Parliament and the British public the last people to know?"