1/100 shot James DeGale lost his IBF super-middleweight belt after he was surprisingly beaten by American Caleb Truax on a majority decision.
De Gale, 31, was making the fourth defence of his title - and fighting on home soil for the first time in more than three years - but the home crowd were left disappointed following a bruising defeat for the former Olympic champion.
One judge scored the fight as a draw at 114-114, but further scores of 115-112 and 116-112 in favour of the challenger - 16/1 for victory at the outset with Sky Bet - condemned DeGale to only the second defeat of his career with Truax crowned as the new champion.
DeGale told BT Sport: "I can't believe that.
"I've boxed at a high level for two-and-a-half years and I've got to go back to the drawing board, I've got to sort certain things out and change things.
"I thought I'd won it but I'll have to look back."
DeGale underwent surgery on his right shoulder following his controversial draw with Badou Jack in January but after a steady start he was rattled in the fifth round when Truax unleashed a number of fierce shots which left blood pouring from his nose.
Asked whether his shoulder problems had contributed to the outcome, DeGale added: "There's no pain there, I've probably rushed back to be honest... maybe I should have waited till next year.
"I'm really gutted, I'm so upset.
"I want the rematch, that's what I want."
The 34-year-old Truax was thrilled to have pulled off his gameplan.
"I want to thank everyone for the opportunity," he said.
"We noticed that he didn't pressure that much, he's a helluva boxer, a classy boxer, I couldn't box because he's a better boxer than me but I had to make it into a dogfight and that's what I did."
DeGale would have been pleased with his first round back in action for nearly a year after he shaded the opener.
But Truax proved he would be no pushover with the following exchanges being edged by the challenger.
The bout then dramatically came alive in the fifth round after Truax unleashed a number of fierce shots against DeGale.
The British fighter was in trouble, up against the ropes and with blood pouring from his nose, but somehow survived and was urging the 34-year-old American to unleash further fury.
DeGale rallied in the closing stages but he headed back to his corner bloodied and in deep trouble.
He steadied the ship in the sixth but his opponent remained in charge of the fight, and as the match headed into the seventh round DeGale appeared in need of a sudden turnaround to retain his belt.
The Londoner put up brave resistance but it was Truax who delivered the more telling shots in the 10th as he caught DeGale with a fierce uppercut.
To his credit, DeGale came out firing in the penultimate round, perhaps sensing that a stoppage would be his best chance of salvaging victory, but Truax stood firm despite encouragement from the partisan crowd.
DeGale shaded the final round, too, but it was Truax who sunk to his knees believing he had done enough to secure a shock win, and so it proved.
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