The Prime Minister has been aware of a Trident missile test that reportedly failed since coming into office in July last year, Downing Street has confirmed.

The Sunday Times reported an unarmed Trident II D5 missile misfired when tested from HMS Vengeance off the coast of Florida in June last year.

Theresa May refused to directly answer four questions on her knowledge of the test on the BBC's Andrew Marr show on Sunday.

The Prime Minister insisted she has "absolute faith in our Trident missiles" .

A Downing Street spokeswoman has now confirmed she did know about the test firing before newspaper reports of it.

The spokeswoman said May had been briefed on the "demonstration and shakedown" operation undertaken by HMS Vengeance on its return to service following a refit.

May was informed the operation was successful, allowing Vengeance and its crew to return to service.

The Prime Minister's spokeswoman said: "The defence secretary and the Prime Minister are routinely informed when one of these specific 'demonstration and shakedown' operations are planned and on the outcome of them.

"In this instance, that was in June so it was under the then-Prime Minister (David Cameron).

"On taking office, the current Prime Minister was briefed on a range of nuclear issues, including this."

She added: "We have been clear that the submarine and the crew were successfully tested and certified.

"That was the purpose of the operation. What is also clear is that the capability and effectiveness of the Trident missile is unquestionable."

The spokeswoman refused to answer if the missile did misfire, maintaining the UK Government's policy of not discussing operational details of nuclear weapons.

A successful test of a UGM-133 Trident II missile from HMS Vigilant was filmed and released by the Ministry of Defence in 2012.

The defence secretary, Michael Fallon, will answer questions on the missile test later on Monday.