"During his time as head of the Ministry of Defence UFO project, Nick Pope was persuaded into believing that other lifeforms may visit Earth and, more specifically, Britain.
His concern is that "highly credible" sightings are simply dismissed.
And he complains that the project he once ran is now "virtually closed" down, leaving the country "wide open" to aliens.
Mr Pope decided to speak out about his worries after resigning from his post at the Directorate of Defence Security at the MoD this week.
"The consequences of getting this one wrong could be huge," he said.
"If you reported a UFO sighting now, I am absolutely sure that you would just get back a standard letter telling you not to worry. ''Frankly we are wide open - if something does not behave like a conventional aircraft now, it will be ignored.
"The X-Files have been closed down." If these words had come from a sci-fi fanatic, they could be easily dismissed by cynics."