Airprox said aircraft safety had been compromised in 26 of the cases and there had been a risk of collision in nine other cases in addition to the three commercial air transport ones.
"As in 1998, more than three-quarters of the civil air transport incidents reported in the first half of 1999 were assessed as involving no risk of collision, " said Gordon McRobbie, director of the UK Airprox Board.
The report said the Airprox board had been of the opinion that the object was unlikely to have been a fixed wing aircraft, helicopter or hot air balloon, given that it had not shown up on radar.