Stanton Friedman,a nuclear physicist, is convinced there have been multiple visitations of Earth by aliens, who will ultimately quarantine us here on Earth.
He said recently declassified CIA files on UFO sightings were all part of the proof.
So why does he say they are here?
Mr Friedman says aliens actually want to prevent humans colonising space and will ultimately quarantine us here for the greater good to prevent mankind travelling.
He said: “I think they are here. I think they are here to quarantine us, keep us from going out there.
“With our track record – we’re evil.”
But Mr Friedman claims our governments are trying to take advanatage of aliens being here by obtaining their technology to achieve world supremacy.
He said: “The first country to replicate [alien] technology will rule the roost if they can build it.
“It’s political too – the US says ‘are we ready to put out anything if the Russians don’t or the Chinese don’t’.”
Mr Friedman, who has written extensively on Aliens and UFOS, has described this culture of secrecy as the world’s “Cosmic Watergate” – a reference to the President Nixon cover-up scandal in the 1970s.
He now tours the globe giving talks on his beliefs at universities and conferences.
Some sceptics are not convinced by his arguments, however.
One poster on the forum skepticforum.com wrote: “Recently I was looking at some of Friedman’s arguments for UFO’s and against sceptics.
“Overall he did a pretty poor job of battling sceptics and proving his point.
“When he says he debunks the debunkers he makes sweeping generalisations and says sceptics ignore some ‘big’ cases even though many of them were looked at by sceptics.
“So my question is what’s his point.
“He only uses old cases that were mostly debunked and just tries to discredit sceptics rather than provide non-speculative evidence.”