British Prime Minister Theresa May has said that she was briefed about the successful certification of a nuclear submarine as she came under increasing pressure over her handling of reports its unarmed Trident missile misfired.

The Sunday Times reported a test firing of the missile from a submarine malfunctioned last June, sending it veering off in the wrong direction, shortly before Mrs May asked parliament to spend £40bn on new submarines.

The report triggered fierce criticism about a perceived lack of transparency from Mrs May and prompted lawmakers to demand answers from her defence minister in parliament.

CNN, citing a US defence official with direct knowledge of the incident, also reported a missile test involving Britain's nuclear deterrent failed off the coast of Florida.

Asked four times during a BBC interview yesterday on whether she knew about the misfire before the vote in parliament, Mrs May repeatedly declined to answer directly. Today, she again failed to give a clear answer.

"I'm regularly briefed on national security issues, I was briefed on successful certification of HMS Vengeance and her crew," she told BBC television, saying the government did not comment on operational details for national security reasons.

The controversy overshadowed the Mrs May's launch of "Modern Industrial Strategy", a central plank of her plans to rebalance Britain's economy as it leaves the EU.

The so-called "demonstration and shakedown operation" happened in June, shortly before Mrs May became prime minister.

In her first major speech to parliament as prime minister the following month, she asked lawmakers to approve the building of four new submarines, a vote that was passed by 472 to 117.

During more than an hour of questions to defence minister Michael Fallon in parliament today, lawmakers criticised the government's handling of the issue, although several said it would not have changed the way they voted.

"While accepting that the nuclear deterrent needs to be shrouded in secrecy, it also needs to deter," said Julian Lewis, a lawmaker in Mrs May's Conservatives and chair of parliament's defence committee.

"Once stories get out there that a missile may have failed, isn't it better to be quite frank about it? Especially if it has no strategic significance?"

Mr Fallon said if the government had had any doubts about the capability or effectiveness of the missiles it would not have asked parliament to vote on them.

Opposition Labour lawmaker and former defence spokesman Vernon Coaker said: "A leak to a Sunday newspaper, followed, frankly, by government stonewalling doesn't enhance support for that deterrent, it undermines it."