Britain's defence ministry was so alarmed by reports of a UFO sighting nearly 20 years ago, it took the unusual step of briefing ministers, according to released secret files.

Other incidents in the files, released by the National Archives, include a woman who claimed she was approached by an alien with a "Scandanavian-type accent".

The group of seven files, released by Britain's National Archives, were recorded between November 1987 and April 1993 by the Defence Intelligence Staff in the Ministry of Defence (MoD) and include details of around 1,200 different UFO sightings.

The MoD was handed negatives of photographs - showing a large diamond-shaped object hanging in mid-air for around 10 minutes before ascending upwards at high speed - taken on August 4, 1990, by two members of the public.

"Such stories are not normally drawn to the attention of ministers," an official noted in a memo included in the file.

"On this occasion, however, the MoD has been provided with six photographic negatives of an alleged UFO by the Scottish Daily Record and has been asked for comments almost certainly for inclusion in a forthcoming story."

The official added that ministers should tell the media that "no definite conclusions (were) reached regarding (the) large diamond-shaped object".

Among the other files released were details of an anonymous female dog walker who said she met an alien who claimed extraterrestrial life was behind crop circles.

The woman, who telephoned the Royal Air Force to relay her experience, said the incident happened on November 20, 1989, while she was walking her dog on a sports field.

She said she was approached by a man who spoke with a "Scandanavian-type accent" dressed in a brown garment that resembled a flying suit who told her that the purpose of the alien visit was friendly.

The files will be free for the public to access for the next month on www.nationalarchives.gov.uk/ufos.