Video filmed in 2009, a year before the Coalition government placed him in Number 10, shows Mr Cameron promising an "open and frank" policy over the question of alien life.
It has raised hopes among so-called disclosure lobbyists that Mr Cameron could even beat US President Barack Obama in making a hoped announcement that intelligent aliens have visited Earth.
Last month Express.co.uk revealed that Stephen Bassett, chief executive of Paradigm Research Group, and the only registered lobbyist in the US on the alien/UFO issue believes Mr Obama will make an historic announcement to the world that aliens exist before he leaves office.
In an exclusive interview at the Express.co.uk offices in London, Mr Bassett said he expected Mr Cameron would quickly follow with a similar announcement.
But the video of Mr Cameron speaking on the issue at a special Q&A event he hosted has buoyed campaigners that he could be the first to break cover and they will be calling on him to make good his pledge.
In answer to a question from the floor, Mr Cameron said no government should be hiding the truth.
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