The 19 different incidents were recorded between 1986 and 1992, and published by the National Archives on its website.

Among the recorded incidents was a letter dated March 1990 from a woman who claimed she was an alien whose spaceship landed during World War II and was recovered by the British military.

"The crashed vehicle contained two males from Spectra, a planet orbiting the star Zeta Tucanae, and a female from one of the two inhabited planets in the Sirius system, Amazon the planet of warrior women," she wrote in the letter, which also included sketches of herself and of Spectrans.

"That female was me," she wrote.

Though the letter did not spark an investigation, another report from an Alitalia pilot did.

On April 21, 1991, the captain of an Alitalia airliner was on its way to London's Heathrow Airport from Milan when it had a close call with a UFO, the newly-revealed documents showed.

"At once I said, 'look out, look out,' to my co-pilot, who looked out and saw what I had seen," Achille Zaghetti said in a report on the incident.

"As soon as the object crossed us I asked to the ACC (area control centre) operator if he saw something on his screen and he answered 'I see an unknown target 10nm (nautical miles) behind you'."

Meanwhile, a local television station had broadcast a story of a 14-year-old boy who said he had seen a low-flying missile disappear that same evening.

Radar images at that time initially labelled the object, "cruise missile??", but it was later confirmed that it was not a military weapon.

But by July 2, however, a defence ministry inquiry found the UFO had not come from any Army firing ranges, and added there had not been any "space-related activity" that night.

All rights reserved. copyright 2018 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.