Theresa May is under pressure to come clean in the House of Commons about the failure of a Trident missile test after MPs accused her of covering up the truth before a crucial parliamentary vote on renewing the nuclear deterrent.
Labour and the Scottish National party said the prime minister must explain herself after it emerged that an unarmed missile malfunctioned during a rare test last June, shortly before MPs passed a motion in favour of replacing four submarines carrying Trident missiles at a cost of £40bn.
They demanded that she appear before MPs at the earliest opportunity after she declined four times to say what she knew during an interview on the BBC’s Andrew Marr Show.
While expressing “absolute faith” in Trident, she refused to go into any details about the incident, which the government said it could not explain for “obvious national security reasons”.
“There are tests that take place all the time, regularly, for our nuclear deterrents. What we were talking about in that debate that took place was about the future,” May said.
Details of the incident were revealed by the Sunday Times, which said Downing Street knew about the error when May stood up in parliament to propose a motion on renewing Trident five days into her tenure at No 10.
It reported that an unarmed Trident II D5 missile – which can kill millions when armed with nuclear warheads – malfunctioned when launched from a British submarine, HMS Vanguard, off the coast of Florida last year. It may have veered back towards the US rather than heading in the direction of west Africa.
Tests are rare as the missiles each cost about £17m. When trials have been carried out in the past they have been publicised, in some cases with accompanying video footage, but this is the first time last year’s exercise has been reported.
In July, weeks after the test and days after May became prime minister, MPs voted overwhelmingly to spend up to £40bn on replacing Britain’s Trident programme.
May drew gasps during the parliamentary debate when she made clear she would be willing to authorise a nuclear strike killing 100,000 people. However, she made no mention of the test.
Jeremy Corbyn, the Labour leader, and Nicola Sturgeon, the SNP leader and Scotland’s first minister, who both oppose renewing Trident, demanded more details about the episode.
Corbyn said it appeared the prime minister chose not to inform parliament about a “pretty catastrophic error”. “Whilst it wasn’t armed, goodness knows what the consequence of that could have been, I think we need a serious discussion about that,” he said.
Sturgeon said: “This is 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.”
Critics of Trident may now seize on the failure and apparent cover-up to argue that the debate about renewing the nuclear deterrent should be reopened.
Labour is expected to push for an urgent question in the House of Commons on Monday, with Nia Griffith, the shadow defence secretary, demanding that May comes to explain herself.
“This report of a Trident missile veering off course during a test is clearly a very serious matter indeed, and we need to know exactly what happened,” she said.
“Furthermore, it is completely unacceptable that today the prime minister chose to side-step questions on the test, and would not even tell us when she knew about the incident.”
Angus Robertson, the SNP Westminster leader, also said May must be held accountable to MPs immediately. “It would be utterly unacceptable, and deeply serious, if it turns out that this information was deliberately kept from MPs at the time of the renewal vote for the Trident weapons of mass destruction programme,” he said.
“Parliament and the public have a right to know if these reports are true … there must be full disclosure about what happened, who knew, when they knew, and why the House of Commons wasn’t informed. Trident is obscenely expensive and morally repugnant. If we now have to add that there is a real possibility it is unreliable and unsafe – then there must be massive question marks about its viability.”
The government has not released further details about the incident despite the political storm, apart from to describe the test as a success. 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 test launch from HMS Vengeance as part of an operation 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.”
HMS Vengeance, one of the UK’s four Vanguard-class submarines designed to carry the UK’s Trident ballistic missiles, returned to sea in December 2015 after a £350m four-year refit programme.
