The cult Scots writer, who also dreamed up ‘Wanted’ and ‘Secret Service’, told of the his bizarre sighting on the social network in the early hours of Thursday morning.
He wrote: “I know this sounds mental, but did anybody else see that huge glowing object over Glasgow last night around 1am? Only 1 mention online.
“We thought it was a firework at first but it never exploded & and just moved from South to North of city over 60 seconds.
“The weird thing was it looked like a wingless plane flying sideways, a wide tube. Like a rolling pin. We all watched open-mouthed.
“This wasn’t a lantern. It went from Ayrshire to Campsies (about 40 miles) in 60 secs. Passed right over us & we ran to other side of house.
“Note: I’m not saying this was a UFO. I think a test plane only explanation. Mach 9 is now possible after all. But odd to see.
“This was bigger than a jumbo jet. Vast as it passed our place and then shot off as orange flare again.
“It also covered 40 miles in a minute, meaning a speed of approx Mach 3 or 4.”
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Twitter user David Wells from Glasgow tweeted him: “Was it orange? I saw 8 or 9 moving in tandem across the sky 4 days ago.”
To which Millar replied: “Yes. Bright orange. Seemed like a flare at first til it got closer and took on odd trajectories, too nimble to be a plane.”
A spokeswoman for Police Scotland said they had not received any reports of UFO sightings.
In December 2012, a packed Airbus A320 about to land in Glasgow came within 300 feet of colliding with mystery object in sky.
The aircraft was over Baillieston when cockpit crew spotted the large “blue, yellow and silver” object.
The unidentified flying object reportedly flew underneath the jet before the pilot had any time to react.
Both the plane’s pilots reported seeing the object 300 feet ahead when the aircraft was on its final approach to Glasgow Airport.
One immediately told air traffic control: “Not quite sure what it was but it’s definitely quite large and it’s blue and yellow.”
He filed a report when he touched down at Glasgow and said the collision risk was high.
An investigation ruled out the presence of any aircraft or stray weather balloons, adding to the mystery.
The incident was recorded in an official report by the UK Airprox Board, who probe near-misses.
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