THERESA May and her Defence Secretary Michael Fallon’s silence on Trident was condemned as “chilling” yesterday after their bid to keep MPs in the dark about a failed Trident missile test was torpedoed by a US official who gave details to CNN.

In a statement to the Commons, Fallon said he could not reveal “operational details” about the test that took place last June because it would jeopardise national security.

Labour, SNP and even his own backbenchers pushed the minister, asking him to explain why they had not been told about the failure before being asked to back the renewal of the £40bn Trident system last July.

But as Fallon spoke in the Commons, CNN published a story, quoting a US defence official “with direct knowledge of the incident”.

The US defence official told the American news network that in June 2016, off the Florida coast, a Trident II D5 missile veered towards the US instead of its intended target, but then ditched into the ocean, an automatic self-destruct procedure when missile electronics detect an anomaly. The test did not involve a nuclear warhead.

On Sunday, May was asked four times while being interviewed by the BBC’s Andrew Marr, if she knew of the failure, which was first reported in The Sunday Times, before the vote. The Prime Minister refused to  answer.

Yesterday, her spokeswoman gave only a little more information, saying the “demonstration and shakedown” test, as it is known, happened in the dying days of David Cameron’s premiership. When May then took over, she was “briefed on a range of nuclear issues”, her spokeswoman said, adding: “This test saw the submarine and crew successfully tested and  certified.”

In Parliament, Fallon tried to dismiss the story, suggesting The Sunday Times had got it wrong and that people should be careful about “believing everything in weekend press” – although he declined to say which part of The Sunday Times’s story was incorrect.

Speaking after the statement, the SNP’s Brendan O’Hara, called Fallon’s refusal to answer questions on the failure “utterly chilling”, and added: “What we heard in the House of Commons was shocking. The response from the UK Government reeked of complacency with no transparency – no clarity and no detail whatsoever on the Trident nuclear missile malfunction.

“There was clearly a withholding of information from parliament about this at the time – and a failure today to answer the key questions about who knew what and when they  knew it.

“The Tories clearly don’t get it. If there was a cover-up before the vote to renew this deadly weapons system, then that is completely unacceptable and if this government doesn’t understand that then that is deeply worrying.”

In Parliament, Labour’s Mary Creagh asked: “The Secretary of State has advised us not to believe everything we read in the Sunday newspapers, but should we believe the [US] official who, while we’ve been sitting here debating, has confirmed to CNN that the 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?”

Fallon once again declined to give “operational details”. Julian Lewis, the Tory chairman of the influential Commons defence committee, said May “should probably have spoken up” about any malfunction during last July’s debate and called for the person responsible for keeping it secret to be sacked.

According to reports, the missile was fired by HMS Vengeance and the fault was down to “telemetric directional data”, which is the information conveyed to the missile needed to hit the target. This could be the result of a human error or because of a fault in the software.

There was little reassurance in hearing that of the 11 tests performed by the UK since 1994, this was the only one not to successfully complete.

Michael Gove said those complaining about the failed test were like “eunuchs complaining about the cost of Viagra”.

The Trident II D5 is generally thought of as a reliable missile and has recorded more than 160 successful tests over the years, most of which have been conducted by the US Navy, who share data gathered with the MoD.

The Royal Navy perform the tests once every three to five years because of the £17 million cost of each missile. That happens more to test the submarines that launch them, which is why the newly refitted HMS Vengeance had the “demonstration and shakedown” last June.

Speaking after a Cabinet awayday in Cheshire, the Prime Minister said: “I’m regularly briefed on national security issues. I was briefed on successful certification of HMS Vengeance and her crew.

“We don’t comment on operational details for national security reasons.”

She added: “The key issue about the debate we had in the House of Commons on the future of Trident is whether we should renew Trident for the future. Should we continue to have an independent nuclear deterrent? I have absolute faith in our independent nuclear deterrent.

“I believe we should continue to have that for the future, the House of Commons voted for that.

“Sadly the leader of the opposition, Jeremy Corbyn, doesn’t want to defend our country in that way.”