The disclosure was in National Archives files concerning reports of unidentified flying objects between 2007 and November 2009.
The latest declassified files released Thursday cover the last two years of work done by the department's UFO desk, the BBC reported.
The 25 files include reports alleging contact with aliens and UFO sightings near British landmarks, and the decision to close the ministry's UFO desk and hotline.
Carl Mantell, a civilian employee with the Royal Air Force's Air Command, suggested in a memo the Defense Ministry significantly reduce the UFO work, saying it was was "consuming increasing resource, but produces no valuable defense output."
Mantell's memo said there was "no defense benefit" in recording, collating, analyzing or investigating the sightings.
"The level of resources diverted to this task is increasing in response to a recent upsurge in reported sightings, diverting staff from more valuable defense-related activities," the memo said.
In the 50-plus years the UFO desk operated, no sighting "has ever revealed anything to suggest an extra-terrestrial presence or military threat to the U.K," it said.
The ministry decided that, effective Dec. 1, 2009, it would shut down the dedicated UFO hotline, answering service and email address.
The declassified files indicated the number of UFO sightings reported to the Defense Ministry tripled in the year the desk closed, the BBC said. Between 2000 and 2007, the ministry received an average of 150 reports per year. But a month before the desk ceased operations, it had already received 520 reports.
The BBC said one possible increase in the reportings could be the popularity of releasing Chinese lanterns during celebrations.