A passenger plane was involved in a near miss with an unidentified object in the skies above Glasgow, according to a report.
Pilots saw a blue and yellow object "loom ahead" as they made their final descent into the city on December 2 last year.
The UK Airprox Board, which investigates near misses, was unable to establish what the crew had seen.
A report by the board said that the Airbus A320 was flying in clear skies above Baillieston just before 1pm when the incident occurred.
The plane was at an altitude of 4,000ft when the pilot and co-pilot reported seeing the object about 100m away.
The report said: "The object passed directly beneath before either of the crew had time to take avoiding action or had 'really registered it', although they were both agreed that it appeared blue and yellow, or silver, in colour with a small frontal area but that it was 'bigger than a balloon'."
The pilot estimated that the distance between the plane and the object had been 300ft and assessed the risk of collision as high. He asked the controller at Glasgow Airport whether anything was being monitored at the time but was told that nothing had been seen on radar.
Further inquiries were carried out into the incident, including a review of radar sources. The report stated: "Investigation of the available surveillance sources was unable to trace any activity matching that described by the A320 pilot. Additionally, there was no other information to indicate the presence or otherwise of activity in the area."
The report said that the object was unlikely to have been a fixed-wing aircraft, helicopter or hot air balloon, as it had not shown up on radar.
It concluded: "Members were unable to reach a conclusion as to a likely candidate for the conflicting aircraft and it was therefore felt that the board had insufficient information to determine a cause or risk."