Hull City owner Assem Allam has been outbid by a mystery fan in the DVLA auction for car number plate HU11 CTY.
The registration plate raised £46,736 at the event in Leeds.
There was so much interest bidding started at £4,600, far above the £900 reserve.
After an initial bidder dropped out at £16,000, the competition came down to Mr Allam's agent and the anonymous Hull businessman.
Mr Allam said: "There was someone from our side who attended the auction because obviously we didn't want it to go outside Hull. They attended and carried on with the bidding against others until it reached £46,000. We wanted to make sure it stayed in the city."
Mr Allam is a fan of personalised plates.
He already owns three – 2 AA, 30 AA and 50 AA, which were bought as investments with money inherited from his father.
But he insisted this one would have been used by the club, not necessarily on one of Mr Allam's own cars.
He said he understood the winner had been prepared to go much higher if bidding had continued.
Lifelong City fan Phil Beaumont went along with a budget of £3,000, hoping for a chance to win.
He said: "I always thought it was going to go for a lot more than I could pay.
"When they started bidding they were going up by £1,000 each time.
"One guy went up to about £16,000 and dropped out, then someone came in and started bidding against a guy in a suit at the back."
Bidding reached £36,000 when the gavel went down to thunderous applause in the auction house.
VAT and other fees increased the total by £10,736.
It was the highest-selling plate so far in a three-day DVLA auction at the De Vere Village Leeds South Hotel.
DVLA events manager Jody Davies said: "It was an extremely thrilling and exciting sale.
"There were a lot of people in the room when it came down to two people locking horns and when the gavel finally went down everybody clapped."
Are you the mystery bidder? Or have you spotted the plate on a car around Hull? Call James Burton on 01482 315251.
