Britain's top UFO expert says there's no smoking gun that proves UFOs exist, but he's seen enough evidence to convert him from skeptic to believer.

Among the evidence, says Nick Pope, who ran the Ministry of Defense's unexplained-encounter section from 1991 to 1994, are:

-- a cigar-shaped object that passed so close to an airliner that the jet's pilot was heard shouting "Look out, look out!";

-- a USAF pilot who caught up to and almost fired upon a UFO over England;

-- a photo of a UFO that British officials worried might actually be part of a secret military project.

There's no government cover-up, Pope makes clear in his posting Sunday on the science blog The X-Journals. But to him, there's enough evidence in the files to convince anyone that more investigation is needed — and that even better material may soon be released.

"The French government's 2007 decision to release its UFO files was a major factor in the U.K. decision," he writes, "as was the fact that the MoD receives more FOI [Freedom of Information] requests on UFOs than any other topic."

In fact, Pope says, the demand for newly declassified UFO files is so overwhelming, the few staffers the Ministry of Defense has on the subject no longer have the time or manpower to follow up on significant encounters.

"Investigations are suffering because of the workload being put on staff due to FOI," he writes, "but FOI is taking priority because if it fails to comply, MoD would be breaching the law."

Hence the decision to start releasing Britain's X-Files en masse in May 2008, followed by a second batch in October.

"MoD has received over 11,000 UFO reports to date and case files on major incidents can run to over 100 pages of documentation," says Pope. "The entire process is likely to take 3-4 years."

Pope acknowledges that most UFO sightings have "mundane explanations" — chiefly misidentification of common natural phenomena and man-made objects.

"But some were more difficult to explain," he says, "including cases where the witnesses were pilots and police officers, together with instances where UFOs had been tracked on radar."

In the airliner near-miss case, an Alitalia MD-80 flying over the English coastline was almost hit by something on April 21, 1991. The case was passed to the Ministry of Defense after Britain's Civil Aviation Authority after no one could identify the other aircraft.

"We launched a full investigation and eliminated all the usual possibilities, including weather balloons, military aircraft, etc. We even checked to see whether we had accidentally fired off a missile of some sort," writes Pope. "We drew a complete blank and the incident remains unexplained to this day."

• Click here to read Nick Pope's full posting at The X-Journals.

• Click here to view the Ministry of Defense's declassified UFO files.

• Click here to visit FOXNews.com's Space Center.