The UK government has been accused of a cover up after failing to disclose that an unarmed nuclear missile may have been mistakenly fired at the US mainland.
The serious malfunction happened last June just ahead of a UK parliament vote on renewing Britain's nuclear deterrent, the Sunday Times has reported.
The paper quoted an unnamed senior naval source as saying the missile may have veered off in the wrong direction towards the American mainland when it was fired off the coast of Florida from a nuclear Trident submarine.
Theresa May has refused to say whether she knew that a Trident missile had misfired just weeks before a Commons vote on the future of the nuclear deterrent.
The Prime Minister declined four times to answer questions about whether she was aware that the launch of an unarmed Trident II D5 missile from a British submarine off the coast of Florida in June malfunctioned.
Downing Street has been accused of covering up the failure, which was reported in the Sunday Times.
Asked repeatedly on the BBC's Andrew Marr show about whether she personally knew about the misfire, Mrs May said only: "I have absolute faith in our Trident missiles."
She added: "I think we should defend our country."
The Government is expected to face questions in the Commons over the issue, with Labour demanding a full inquiry.
May's denial will prompt questions over her decision to force a Commons vote on Trident renewal just weeks after the alleged failure.
It has been reported that the cause of the failure remained top secret but questions could be asked over the Government's failure to publicise the failed test weeks before MPs approved the £40 billion Trident renewal programme in July.
There have been claims that when fired the missile went in the wrong direction, heading towards the United States.
Previous tests have been publicised by the Government.
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 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."