Mr Icke, 53, claims the Queen and the Duke of Edinburgh are shape-shifters who drink human blood to look like us.
And the father-of-three says a race of half-human, half-alien creatures has infiltrated all the world's key power positions.
He claims the US president, George W Bush, and his father, the former president, George Bush, are both giant lizards who change into humans.
Mr Icke, a professional speaker who has published 16 books, believes that the alien hybrids were behind the "murders" of Princess Diana and John F Kennedy, as well as the terrorist attacks of 9/11.
He claims the only reason that the public cannot see this is because we are obsessed by popular culture, such as EastEnders and Coronation Street, and Page Three girls.
Mr Icke retired from public life after being ridiculed for appearing on the Wogan TV chat-show in 1991 and claiming he had been chosen as the Son of God. The former Green Party spokesman now lives on the Isle of Wight with his second wife, Pam, and lectures around the world on his theories.
He was speaking about them on The World's Strangest UFO Stories - The Great Alien Conspiracy on the Discovery Channel last night.
Mr Icke said: "When you get back into the ancient world, you find this recurring theme of a union between a non-human race and humans - creating a hybrid race.
"From 1998, I started coming across people who told me they had seen people change into a non-human form. It's an age-old phenomenon known as shape-shifting. The basic form is like a scaly humanoid, with reptilian rather than humanoid eyes."