Theresa May 'faith' in Trident after test 'malfunction'
Theresa May says she has "absolute faith" in the UK's nuclear weapons system despite reports that an unarmed missile went off course during a test.
The Sunday Times says the missile, fired in June, veered off course, weeks before a crucial Commons vote on Trident's future.
When questioned by the BBC, Mrs May repeatedly refused to say if she knew about the misfire ahead of the vote.
Labour said it was "extremely worrying" and "questions have to be asked".
Meanwhile Scotland's First Minister Nicola Sturgeon tweeted that the misfire was a "hugely serious issue".
"There should be full disclosure of what happened, who knew what/when, and why the House of Commons wasn't told," she said.
When asked by BBC's Andrew Marr if she knew the misfire had happened Mrs May said: "I have absolute faith in our Trident missiles. When I made that speech in the House of Commons, what we were talking about was whether or not we should renew our Trident."
She was asked a further three times but she did not answer the question.
The Ministry of Defence did not give details of the test process but said it was a success.
Labour former defence minister Kevan Jones has demanded an inquiry into the claims, telling the Sunday Times: "The UK's independent nuclear deterrent is a vital cornerstone for the nation's defence.
"Ministers should come clean if there are problems and there should be an urgent inquiry into what happened."
Labour's Shadow Chancellor John McDonnell told Andrew Marr on Sunday he would have expected the incident to be reported to Parliament.
"The fact that Theresa May didn't report it to Parliament is extremely worrying and questions have to be asked about it."
According to the Sunday Times, it is expected that Defence Secretary Michael Fallon will be called to the Commons to answer questions from MPs.
In July, MPs backed the renewal of Trident by 472 votes to 117, approving the manufacture of four replacement submarines at a current estimated cost of £31bn.
The Sunday Times says the cause of the test firing failure from HMS Vengeance remains top secret but quotes a senior naval source as saying the missile suffered an in-flight malfunction after launching out of the water.
The Trident II D5 missile, adds the paper, was intended to be fired 5,600 miles (9,012 km) to a sea target off the west coast of Africa.
HMS Vengeance, one of the UK's four Vanguard-class submarines, returned to sea for trials in December 2015 after a £350m refit, which included the installation of new missile launch equipment and upgraded computer systems.
BBC defence correspondent Jonathan Beale said while the MoD has described the test as a success for the crew and the boat, it has not denied the report that the missile itself might have veered off course.
In the past the MoD has issued a press release and video of successful tests but its silence on this occasion has raised questions as to whether any fault was deliberately kept quiet ahead of the key vote, our correspondent added.
Labour's official policy is to support renewing the Trident system, but leader Jeremy Corbyn – a longstanding opponent of nuclear weapons – wants to change the party's position and has launched a defence review to examine the issue.
A statement issued by both Downing St and the MoD says: "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."
What is Trident?
The Trident system was acquired by the Thatcher government in the early 1980s as a replacement for the Polaris missile system, which the UK had possessed since the 1960s.
Trident came into use in the 1990s. There are three parts to it – submarines, missiles and warheads. Although each component has years of use left, they cannot last indefinitely.
The current generation of four submarines would begin to end their working lives some time in the late 2020s.