Brad Jayakody, 30, was told he had to change his T-shirt if he wanted to catch his flight from Heathrow's Terminal 5.
His top showed the Transformers film character Optimus Prime holding a gun.
Mr Jayakody, an IT consultant, was scheduled to board the British Airways plane to Dusseldorf, Germany, with four work colleagues.
"My mate set off the alarms and was searched. But then the guy told me to stop and said 'you cannot get on the plane because there is a gun on your T-shirt'" he said.
Mr Jayakody, from Bayswater, West London was then threatened with arrest after he asked to see the security chief.
He said: "It's a cartoon robot with a gun as an arm. What was I going to do, use the shirt to pretend I have a gun?
"I was flabbergasted. I thought the supervisor would come over and see sense, but he didn't. After I changed he said if I changed back I would be arrested."
A spokesman for Heathrow operator BAA said: "If a T-shirt had a rude word or a bomb on it for example, a passenger may be asked to remove it.