London - None of the numerous UFOs reported over Britain in the last 30 years was a flying saucer, the government said Monday as it released previously secret defence files probing mysterious aerial sightings.

The declassified study - "Unidentified Aerial Phenomena (UAP) in the UK Air Defence Region" - concluded that such UAP do exist and are "usually described as coloured lights and sometimes as shapes".

"Very occasionally they are reported with sound and even with smell," it added.

But it noted that "reports occur because they comprise unfamiliar and unexpected lights, shapes and patterns, in the context in which the observer sees them. The phenomena occur on a daily, worldwide basis".

Man-made objects, such as airplanes, are also often mis-seen or are actually meteors re-entering the Earth's atmosphere causing "bouyant plasmas".

Rare weather, atmospheric conditions or other "barely understood" natural events could also be causes.

"There is no evidence that any UAP seen in the UKADR are incursions by objects of any intelligent (extra-terrestrial or foreign) origin, or that they represent any hostile intent," the unnamed author of the report said.

"There is no evidence that 'solid' objects exist which could cause a collision hazard."

The study, which aimed to assess whether Britain was threatened by UAPs and "should the opportunity arise, to identify any potential military technologies of interest", concluded there was "nothing of defence intelligence value".

But it warned Royal Air Force pilots, who are most likely to encounter such phenomena, not to try to intercept UAPs because of the threat of collision.

The report, marked "Secret. For UK Eyes Only", was made public following a request under the Freedom of Information Act. - Sapa-AFP