Ever heard of a Skrull? No? Not a comic book fan then? Well, the Skrulls are a Marvel Comic invention. They're not very nice and they're shapeshifters.
More importantly, in the latest issue of Captain Britain and MI:13, there is a guest appearnce from Gordon Brown. Apparently, three of his Cabinet turn out to be shapeshifters as well.
Feel free to speculate on which memebrs of the Cabinet you currently think might actually be secret aliens sent to infiltrate earth in preparation for invasion.