The British government has been accused of covering up a failed test of its nuclear weapons deterrent last year, just weeks before MPs 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 US.

"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 18 July to approve the replacement of the ageing submarines that carry Britain's nuclear arsenal.

Ms May was not prime minister at the time of the test, but she took office shortly before the vote and successfully appealed to MPs to approve the £41 billion (€47bn) project.

In a BBC interview today, 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 US and France.

SNP leader Nicola Sturgeon tweeted: "This is a hugely serious issue. There should be full disclosure of what happened, who knew what/when, and why House of Commons wasn't told." 

Kate Hudson, general secretary of the Campaign for Nuclear Disarmament, said the failed test "would have impacted on the debate in Parliament on Trident replacement". 

"So the Government's motivation for holding back this vital information is clear," she added. 

"Instead this crucial information has been revealed by a senior naval figure rather than by Government at the appropriate time to inform the Parliamentary debate.

"This is shocking behaviour on the part of our Government and it is profoundly to be hoped that Parliamentary opposition forces will hold Government to account for withholding information that is crucial to our national security."