A test firing of an unarmed British nuclear Trident missile from a submarine malfunctioned last June, the Sunday Times reported, prompting questions about why Prime Minister Theresa May did not tell parliament ahead of a vote on renewing the submarines.

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.

It was the only test firing of a British nuclear missile in four years and came shortly before May became prime minister in the wake of Britain's vote last June to leave the European Union, the paper said.

It said May had omitted any mention of the failed test when she persuaded parliament to spend 40 billion pounds ($49.5 billion) on new Trident submarines in her first major speech to parliament as prime minister last July.

Asked four times during a BBC television interview on Sunday whether she knew about the misfire before she made that speech, May repeatedly declined to answer directly.

"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, whether or not we should have Trident missiles," she said.

"There are tests that take place all the time, regularly for our nuclear deterrent."

The Times said Trident missiles have been test-fired only five times by UK submarines this century because they each cost 17 million pounds.

After more than five hours of debate, parliament last year voted to rubber stamp a 2007 decision to replace the deterrent system, approving the building of four submarines to ensure Britain can have nuclear weapons continuously on patrol at sea.

But more than 100 of parliament's 650 lawmakers voted against doing so, with the Scottish nationalists and some opposition Labour members arguing the weapons were no longer needed as they are little use against terrorists and the money could be better spent elsewhere.

Labour leader Jeremy Corbyn and his finance spokesman John McDonnell, who both oppose Trident, on Sunday called for a discussion on the reported misfire.

"People on both sides of the argument on Trident would have expected that to be reported to parliament and the fact that Theresa May didn't is extremely worrying and I think questions have to be asked about that," McDonnell told the BBC.

In a joint statement, May's office and Britain's Ministry of Defence said the Royal Navy conducted a routine unarmed Trident missile test in June from HMS Vengeance, as part of an operation designed to certify the submarine and its crew. It did not confirm the failure.

"Vengeance and her crew were successfully tested and certified, allowing Vengeance to return into service. We have absolute confidence in our independent nuclear deterrent," the statement said.

"We do not provide further details on submarine operations for obvious national security reasons." ($1 = 0.8083 pound)