David Cameron said he "was convinced" the Earth had been visited by aliens has vowed to publish any secret files that may exist on UFOs if he becomes prime minister.

Mr Cameron, speaking at one of his regular Cameron Direct meetings, where he takes the public's questions, joked that Business Secretary Lord Mandelson was one of the alien beings.

The Conservative leader promised to be "entirely open and frank" about what the Government knows about close encounters after he was questioned about a spate of mysterious incidents at the meeting in Tynemouth, on Tyneside.

A member of the audience asked: "In July last year the respected scientist and astronaut Dr Edgar Mitchell, who was the sixth man to walk on the moon, spoke on a British radio station.

"He said the American government had had contact with extra terrestrials on multiple occasions and that these were ongoing.

"He spoke about the Roswell event in 1947 where wreckage of a downed UFO was recovered and found to contain alien bodies.

"He said this event was real but was covered up by the Government for many years.

"Do you agree with me that the British people have a right to know if we have been visited, and if so, when you become prime minister will you seek to lift the veil of secrecy and give the public the truth that they deserve and that has been covered up for all these years?"

Mr Cameron laughed and replied: "I'm convinced we have been visited by alien life forms - and one of them is the trade secretary Peter Mandelson."

He added: "I have no idea there is intelligent life out there and no idea whether any of the sightings which have taken place or whether any incidents which have taken place have any basis in truth.

"I do believe in freedom of information and openness and this question has been asked from time to time and I think we should be as open and clear as possible.

"What has tended to happen when people have looked at the Roswell incident, or when people have looked at pictures, is a rational explanation tends to be produced to try and show what has happened is not what those who believe in UFOs suggest.

"But I think we should be a open as possible, so I would be quite happy to give you a guarantee that if I became prime minister I would always be entirely open and frank about these things.

"I don't think any of us have any clue whether there's intelligent life out there and it is certainly not something that any Government should seek to hide from anyone."

He apologised to the man for the joke.