The UFO was said to hover over the stadium for 15 seconds
A police officer saw an unidentified flying object hovering over Chelsea FC's stadium, according to newly released Ministry of Defence files.
The officer described seeing four diamond-shaped lights over Stamford Bridge in south-west London on the evening of 10 March 1999.
The lights, also seen by his colleague, were said to "move across the sky fairly quickly, changing shape slowly".
The officer said the lights were "not like anything seen before".
The sighting was among more than 6,000 pages of reports describing people's experiences with unidentified flying objects in the UK between 1994 and 2000.
The lights reportedly moved from east to west as the officer, from Chesham, Buckinghamshire, stood with his horse.
Although it was a clear night he was unable to state the size of the object, which was said to make no sound.
The details have been released under a three-year project between the Ministry of Defence and The National Archives.