But the Prime Minister’s official spokeswoman declined to confirm or deny reports that a missile malfunctioned during the test, stating only that Mrs May was told the operation was completed successfully.
Defence Secretary Sir Michael Fallon is to face a grilling from MPs on the incident on Monday afternoon, after Commons Speaker John Bercow granted an urgent question from Labour backbencher and former defence minister Kevan Jones.
During a TV interview this weekend, Mrs May four times ducked questions about Sunday Times reports that an unarmed Trident II D5 missile veered off course after being launched from a Royal Navy submarine off the coast of Florida in June 2016.
But her official spokeswoman told reporters the PM was briefed on the “demonstration and shakedown” operation undertaken by HMS Vengeance.
The PM was informed the operation was successful, allowing Vengeance and its crew to return to service following a refit.
Just days after becoming Prime Minister in July 2016, Mrs May set out the case for renewing the Trident system in a speech in the Commons in which she did not mention the previous month’s test.
Asked whether Mrs May was told about the result of the test before making the speech, the PM’s spokeswoman said: “The Defence Secretary and the Prime Minister are routinely informed when one of these specific ‘demonstration and shakedown’ operations are planned and on the outcome of them.
“In this instance, that was in June so it was under the then prime minister (David Cameron). On taking office, the current Prime Minister was briefed on a range of nuclear issues, including this.”
The spokeswoman declined to say whether Mrs May was informed of a malfunction in the missile system, stating it was not Government policy to discuss operational details of tests in public, and telling reporters that she did not anyway “accept the premise of the question”.
“We have been clear that the submarine and the crew were successfully tested and certified,” said the spokeswoman. “That was the purpose of the operation.
“What is also clear is that the capability and effectiveness of the Trident missile is unquestionable.”
Both Mrs May’s spokeswoman and Business Secretary Greg Clark said it was the long-standing policy of the Government not to comment on the tests of weapons systems.
But their position appeared to be undermined by a string of previous occasions on which the Ministry of Defence has openly discussed successful tests.
In October 2012, the MoD released a press statement stating that then defence secretary Philip Hammond was announcing extra funding following “the successful firing of an unarmed Trident ballistic missile by HMS Vigilant during a test launch in the Atlantic Ocean last week”.
The test, undertaken during the Vigilant’s own demonstration and shakedown operation, was mentioned by senior MoD official Bernard Gray later that year in a Government magazine, which carried pictures of the launch.
And in a speech in June 2014, then defence procurement minister Philip Dunne said: “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 the Trident remains a credible and reliable deterrent.”
The chairman of the Commons Defence Committee, Conservative MP Julian Lewis, said that if a botched test was covered up, those responsible should lose their jobs.
Dr Lewis told Today: “These tests are routine but infrequent in this country. Whenever they work, which is 99% of the time, film is released of them working.
“So whichever person decided they wanted to draw a veil over one that didn’t work should have been sacked.”
Foreign Affairs Select Committee chairman Crispin Blunt, who opposed Trident renewal, said he believed Mrs May would have been told all the details of the test and MPs should also have been informed.
The senior Tory told BBC Radio 4’s World At One: “Everyone is told if these things succeed because these things are normally a bit of an occasion with lots of VIPs watching the missile emerge from the submarine and everyone congratulating themselves on a successful test firing.
“That’s certainly been the case in the last 20 years and suddenly everybody went frightfully quiet after this firing.”
Former defence minister Sir Julian Brazier told the programme: “We don’t know if this is the only time there’s been a failure, although it is very strongly believed it is the first time there has been a failure on this.
“This is a good system. If some parts may need renewing that’s exactly why we do tests.”
Asked if knowledge of a failed test could have influenced MPs’ decision to approve Trident renewal, shadow defence secretary Nia Griffith told BBC Breakfast: “We can’t speculate on that until we have a full report, and that’s what we’re calling for today.
“We need to have the full detail of exactly what did happen and why this occurred.”