Theresa May did know about last year’s controversial test firing of a Trident missile, but No.10 is refusing to confirm that it veered off course.

Instead, the Prime Minister’s spokeswoman insisted the operation had been “successful” – because both the submarine and the crew were able to return to service.

Her spokeswoman described repeated questions about allegations that the unarmed missile went astray as “minutiae and specifics”.

Theresa May dodges question four times over Trident misfire

The admission that Ms May was informed about the results of last June’s test comes 24 hours after she refused – four times – to say if she had been aware of it.

She has been accused of covering up the test, which came just weeks before MPs backed the £40bn renewal of Trident by 472 votes to 117.

Caroline Lucas, the Green MP, immediately tweeted: “Absolutely no excuse now for Theresa May dodging Parliament on this. She must come in today, answer questions and apologise.”

Defence Secretary Michael Fallon has been hauled back to the Commons from a Cabinet trip to Lancashire and will face a grilling from MPs this afternoon.

In the past, successful tests have been publicised by the Ministry of Defence, which has even released videos to show off Trident’s effectiveness.

According to the Sunday Times, the missile was intended to be fired 5,600 miles (9,012 km) from the coast of Florida to a sea target off the west coast of Africa - but veered towards the US.

But, asked about the story by the BBC’s Andrew Marr on Sunday, the Prime Minister repeatedly refused to say if she knew about the alleged malfunction.

Today, No.10 changed course, telling journalists that Ms May had – after becoming Prime Minister, in July – been briefed about the test and that it had been “successful”.

It was a “demonstration and shakedown” operation undertaken by HMS Vengeance following a refit and both submarine and crew had been “certified” to return to service.

Asked if the Prime Minister was told the missile had veered off course, her official spokeswoman replied: “I don't accept the premise of the question.”

She said: “We have been clear that the submarine and the crew were successfully tested and certified. That was the purpose of the operation.

“What is also clear is that the capability and effectiveness of the Trident missile is unquestionable.”

Asked, again, if there had been a “malfunction”, the spokeswoman replied: “I’m not going to get into operational details.”

Labour and other senior MPs had been told about the test – but the spokeswoman could not say if they were told the results of it.

Earlier, Business Secretary Greg Clark told the BBC: “It's been the longstanding policy not to comment on tests of weapons systems and I think if that's the approach you take we have to abide by that.”

Yet, in 2012, a Ministry of Defence press release referred to “the successful firing of an unarmed Trident ballistic missile by HMS Vigilant during a test launch in the Atlantic Ocean last week”.

And, in 2014, another quoted then-defence minister Philip Dunne saying: “Last week I was off the coast of Florida embarked on USS West Virginia to witness a test firing of 2 Trident 2 D5 missiles.

“This successful test demonstrated once again that that Trident remains a credible and reliable deterrent.”

Nia Griffith, Labour's shadow defence secretary, has demanded that the Government give “a full explanation” to MPs later.

And Nicola Sturgeon, the Scottish First Minister, where the submarines are based, on the River Clyde, called the apparent misfire a “hugely serious issue”.