The British government was accused on Sunday of covering up a failed test of its nuclear weapons deterrent last year, just weeks before lawmakers voted to renew the system.

Prime Minister Theresa May refused to say whether she knew about the reported malfunction of an unarmed missile when she urged MPs to support updating the Trident nuclear system.

The Sunday Times newspaper, citing a senior naval source, claimed that the Trident II D5 missile failed after being launched from a British submarine off the coast of Florida in June.

The cause of the failure is top secret but the source suggested the missile may have veered off in the wrong direction towards the United States.

"There was a major panic at the highest level of government and the military after the first test of our nuclear deterrent in four years ended in disastrous failure," the source told the paper.

"Ultimately Downing Street decided to cover up the failed test. If the information was made public, they knew how damaging it would be to the credibility of our nuclear deterrent."

The malfunction came just weeks before the House of Commons was asked on July 18 to approve the replacement of the ageing submarines that carry Britain's nuclear arsenal.

May was not prime minister at the time of the test, but she took office shortly before the vote and successfully appealed to lawmakers to approve the 41 billion pounds (47 billion euro, $50.7 billion) project.

In a BBC interview on Sunday, she sidestepped questions about whether she knew about the malfunction when she made her statement to MPs.

"What we were talking about is whether or not we should renew Trident," she said.

"I have absolute faith in our Trident missiles," she continued, adding that tests take place "regularly".

Opposition Labour leader Jeremy Corbyn, a longstanding opponent of nuclear weapons, said it was a "pretty catastrophic error" for a missile to go in the wrong direction.

A government spokesman confirmed the Royal Navy conducted a routine test launch of an unarmed missile last June from HMS Vengeance, one of Britain's four nuclear-armed submarines.

It was "part of an operation which is designed to certify the submarine and its crew", he said.

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

Britain is one of only three nuclear-armed NATO nations, along with the United States and France.