Cameron will reveal all on UFOs

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

The man added: "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 then said: "I have no idea (if) 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.

"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."

Mr Cameron apologised to the man for the joke.