Amir Khan went down to a shock points defeat by Lamont Peterson to lose his WBA and IBF light-welterweight titles in a pulsating fight in Washington DC.
Two points deducted from Khan for pushing proved crucial as the judges scored the fight at 113-112, 113-112, 110-115 in favour of the local fighter.
The Briton knocked Peterson down in the first round, but the American launched huge attacks in the third and seventh.
Khan recovered, but the second lost point in the final round was decisive.
"It was like I was against two people in there - the referee and Lamont himself," complained Khan immediately after the decision.
"Every time he was coming into me he kept putting his head low. I was the cleaner fighter - he was so wild in there.
"I had to push him away because his head was coming so low."
Talk before the fight centred around the 25-year-old Khan's desire to move up to welterweight, with the Bolton fighter frustrated in his attempts to secure light-welterweight unification bouts with WBC champion Timothy Bradley and WBO title-holder Erik Morales.
The former Olympic silver medallist has spoken of his wish to meet Floyd Mayweather at 147lbs but those hopes now seem set to be placed on hold after this demoralising defeat.
Despite talking of his longer-term plans before this bout, the overwhelming favourite Khan insisted he would not underestimate Peterson and, as if to prove his point, the champion exploded into action at the opening bell.
Twice Peterson, 27, hit the canvas in the first round, initially after taking a left when appearing to have slipped, then after a lightning combination from Khan which drew the referee's count.
But that would be the last time that Khan would use his superior speed to such effect as, from the third round onwards, Peterson succeeded in cutting the distance between the fighters and took advantage of his greater power.
Khan's response was to go toe-to-toe with the home-town challenger, but too often was forced to retreat as Peterson played the aggressor.
The champion recovered and, even though he hit the canvas after slipping in the fifth, he had the better of that and the following round.
Back came Peterson, catching Khan on the ropes with right and left hooks, and it was to get worse for Khan as he was deducted his first point.
The champion took more punishment, but was able to respond in a frantic ninth round. A left-right combination rocked Peterson only for the challenger to again display the resilience of a man who was once a homeless youth living on the streets of the United States capital.
He marched forward in the 10th, but the Briton shaded the 11th and there seemed little to separate the two men as the final round got under way.
However, the final twist came when Khan, after numerous warnings, lost another point and, following a lengthy wait to hear the decision, the loss of his titles was confirmed.
The unfancied Peterson, in the bout as the mandatory IBF challenger, said: "I was the big underdog - a lot of people thought Khan was going to win.
"Early on they were probably saying it again, but I'm not going to lay down for anyone, I'll fight the whole way through.
"I would definitely give him a rematch - he gave me a chance to fight for a world title, why not? It was a good fight, I'm pretty sure everybody enjoyed it and I don't mind doing it again."