Nick Pope, who is now based in the US, worked for the Ministry of Defence on its now closed investigations into the UFO phenomena from 1991 to 1994.
He said no world government is prepared at all for a potential alien invasion, reports Metro.co.uk.
Speaking ahead of the March 20 DVD release of the sci-fi movie Arrival, about an alien visitation of Earth, Mr Pope added: “There’s no contingency plan as far as I’m aware.
“The government’s answer would be that, ‘We regard this as an incredibly small possibility’.
“But I would say, why not have a plan if there’s even the smallest possibility of it happening.”
Mr Pope is not suggesting a War of the Worlds-style invasion is imminent, but said teams of scientists and military experts should be ready to communicate with any new arrivals.
He said this was needed because if aliens were able to visit Earth from a far-flung corner of the galaxy, they would be far superior to us technologically.
And even a peaceful alien visitation could create an inter global conflict with world super powers trying to get hold of alien technology to ensure they stayed on top.
He said: “There would be a scramble for the alien technology, and weapons.
There would be a scramble for the alien technology, and weapons.
“Some of the big countries – and also, nowadays, corporations – would be scrambling to get their hands on alien technology.
“The country that gets that [technology] would be the world leader for the foreseeable future.”
But, he say, it may instead unite the world.
He added: “Ultimately this [an alien visit] might have a unifying effect.
“An extra terrestrial arrival might convince us that there’s more that binds us together than pushes us apart.”
“Don’t start running for the hills or shooting at alien ships.
“Wait for some of the finest minds on the planet to figure out what this is.”
Astronomers have found several exoplanets which orbit stars like ours and are the right distance from them to have the potential conditions for life to form.
But, so far, there is no confirmed evidence that aliens exist, but many conspiracy theorists claim world governments already have evidence and keep it hidden from the public under an alleged truth embargo amid fears of the impact the knowledge would have on religion and the rule of law.