David Clayton, who is an expert on crop circles and animal mutilations, both allegedly carried out by visiting aliens, told the UFO Truth Magazine international annual conference of a sinister plot by spooks to ensure the truth never gets out.

Mr Cayton told of seeing crop circles in the 1980s that could not have been made by hoaxers, and therefore must have been alien in origin, because the plants within the circle had not been flattened by physical force, but by heat.

There are many UFO believers who claim some crop circles are what is left after a flying saucer lands in a field.

Mr Clayton admitted that most crop circles that had been highlighted since the 1990s were fakes, but real ones had been found.

He claimed British intelligence agencies were behind the bulk of fake crop circles to make genuine research seem ridiculous.