The Prime Minister Theresa May faces tough questions in the commons following revelations that she knew about the failure of a Trident missile test and covered it up ahead of a vote to renew the defence system in July 2016.

The Trident system consists of four Vanguard-class submarines which can carry up to 16 Trident II D5 ballistic missiles, each armed with up to eight nuclear warheads. At any time, one submarine is on patrol, one is undergoing maintenance, one is preparing for patrol and one has just come off patrol and is recovering.

It has been Britain’s chief nuclear deterrent for over 40 years and requires regular testing and renewal to maintain its combat effectiveness.

As the last part of a process of certification to allow HMS Vengeance to resume service, the submarine test-fired a Trident II D5 ballistic missile off the coast of Florida.

It was aimed at the southern Atlantic off the coast of Africa but headed off in the opposite direction over the US and the test was aborted. In spite of the obvious malfunction, HMS Vengeance was certified and resumed  naval service in June 2016.

Five days after becoming Prime Minister, the commons voted overwhelmingly to replace Britain’s Trident nuclear weapons programme. More than half of Labour MPs join Conservatives to pass it by a majority of 355, at a cost of more than £40 billion.

Mrs May, who appeared on the Andrew Marr show on Sunday, was repeatedly questioned whether she knew about the malfunctioning missile test before the commons vote but refused to comment (despite being asked four times).

Downing Street was later forced to admit that the Prime Minister had been fully briefed on the failure of the missile test but would not comment on whether she purposefully withheld the information from MP’s ahead of the commons vote.

Labour were granted an urgent question on Trident in today’s commons and took the opportunity to press the defence minister Sir Michael Fallon for more details about the test and whether the government had engaged in a cover up.

In a statement to the commons he told MPs: “Contrary to reports in the weekend press, HMS Vengeance and her crew were successfully tested and certified as ready to rejoin the operational cycle.

“We do not comment of the detail of submarine operations.”

He added: “The capability and effectiveness of the UK’s independent nuclear deterrent is not in doubt. The Government has absolute confidence in our deterrent and in the Royal Navy crews who protect us.”

With the government refusing to comment on these potentially damaging allegations, we can expect further tough questioning in the days to come.