"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.

But Mr Pope's CV - he was head of the UFO project between 1991 and 1994 - cannot be ignored.

When he began his job, he too was sceptical about UFOs but access to classified files on the subject and investigation of a series of spectacular UFO sightings gradually changed his mind.

And while Mr Pope says that there is no evidence of hostile intent, he insists it cannot be ruled out.

"There has got to be the potential for that and one is left with the uneasy feeling that if it turned out to be so, there is very little we could do about it," he said.

"If you believe these things are extra terrestrial craft then you cannot rule out that what is happening is some kind of covert reconnaissance."

One incident which persuaded him of the existence of alien lifeforms was in 1993. There were reports of a "vast, triangular-shaped craft" spotted flying over RAF bases in the West Midlands.

"Most of the witnesses were police and military personnel," he said.

"Hundreds of members of the public also had sightings over a period of several hours."

In another incident in 1980 at RAF bases in Suffolk, staff investigated a suspected plane crash after bright lights were reported coming from nearby woods.

They found a kind of lunar landing module standing on three legs which then flew off. The indents it left in the ground were found to emit ten times the normal levels of radiation. Mr Pope said: "These sort of incidents are why I got so frustrated.

"In my time I would brief the more interesting sightings up the chain of command to people like the Chief of the Air Staff and would get the answer back that it was very interesting and I had clearly done a good job investigating it and that was it.

"Every one is a piece of a puzzle but no one takes it seriously. There needs to be more resources and people who are prepared to look past the philosophical issues, look at the reports and investigate them properly.

"Whether you believe these things are foreign air forces testing prototype aircraft or whether you believe they are something more exotic, with the speeds and movements they are capable of, it's technology we would very much like to get hold of."

A spokesman for the Ministry of Defence insisted that all UFO sightings were investigated for "evidence to suggest that UK airspace has been compromised by hostile or unauthorised air activity."

She said: "Unless there is such evidence, the MoD doesn't attempt to positively identify what was seen."

Mr Pope is continuing his UFO research in a private capacity since leaving the MoD and is recognised as a leading authority on UFOs and the unexpected.

He has written four science fiction books drawing on his experience at the MoD, and lectures around the world on the subject.

He has appeared on BBC Newsnight and Radio 4's Today programme and has acted as consultant on numerous television documentaries.

Also asked to advocate for the swap were leftist academics Noam Chomsky, James Petras and Angela Davis and activist Jesse Jackson.

"To the people of the United States, we ask for your always generous solidarity to pressure President Bush and his government to support a prisoner exchange in Colombia," said Raul Reyes, the chief spokesman for the FARC.

Both directors have made political films in the past which might explain their inclusion on the list: Stone with Salvador and Moore with Fahrenheit 9/11. Likewise Chomsky was recently promoted by Venezuelan President Hugo Chavez at a recent meeting of the United Nations.

The inclusion of Washington, one of Hollywood's most prominent actors, is puzzling, though in his last movie role he played a police detective attempting to negotiate with a bank robber in director Spike Lee's The Inside Man and has played activists such as South Africa's Steve Biko and U.S. leader Malcolm X in films.

FARC is holding three U.S. defence contractors - Thomas Howes, Keith Stansell and Marc Gonsalves - whose small aircraft went down in 2003 while intelligence gathering. Also among the hostages is former Colombian presidential candidate Ingrid Betancourt.

"Howes, Stansell and Gonsalves are alive in our custody, treated with respect and dignity in the jungle," said Reyes, who promised to send evidence the three were alive.

Colombian President Alvaro Uribe broke off negotiations with the group after claiming the group was responsible for a car-bomb that injured 20 people. The government said they intended to use military operations to free the hostages but later relented after pressure from family members of the hostages.

Among the rebels the group wants released is Ricardo Palmera, also known as Simon Trinidad, who was captured in 2004 and stands trial in Washington, D.C., for the kidnapping of the three Americans.