In an interview with MSN, Nick Pope, who worked for the Ministry of Defence for 21 years, said that while Britain doesn't have a war plan, it certainly had sophisticated enough weaponry to defend itself.
Mr Pope, whose job it was to advise on the threat posed by other life forms, said that in the event of an attack he expected that we would "quickly adapt our plans for other more current war scenarios."
At present, he said, there was no special organisation like Torchwood, the alien-hunting team from the BBC science fiction series of the same name, but he said he was sure that in the event of attack an operation could be set up quickly and efficiently.
World nations would probably pool their technology together like in films such as Independence Day.
Mr Pope said: "We do have several prototype aircraft and drones and other weapons you won't see on the news for another 10-15 years so if we did face a threat from the unknown then even if there is no Torchwood around now, there would be something like it by then and they certainly would have some great kit to help in the fight."
He said: "Look at the Taranis, which is a prototype made by BAE Systems. It looks for all the world like a spaceship in the hanger."
>> So are aliens a threat?
The official government position is that UFOs offer "no significant defence threat".
However, Mr Pope said what that really means is that "we don't know".