Amir Khan was knocked out inside the first minute of his WBO intercontinental lightweight title defence by Colombia's Breidis Prescott.
The fight was over in less than a minute as Prescott blew Khan awayKhan was put down for the first time after just 30 seconds when he was hit by two lefts in quick succession.
The 21-year-old managed to regain his feet but was clearly struggling.
He resumed after an eight count but Prescott unleashed another flurry of punches and Khan hit the deck once more before being counted out.
It was the first defeat for Khan in the 19th fight of his career.
"I made a little mistake in there and couldn't get it back again. It was a stupid mistake," said Khan.
"That's boxing. One punch can change the fight. I'm fine but I need to go back to the drawing board and put it right.
"I'm a world beater and I'm going to come back stronger. I'm really sorry to my fans."
Going into the fight there were suggestions that the big-punching Prescott would provide a stern test of Khan's much-criticised defence, and he had rocked the Bolton fighter within seconds of the start.
A stiff jab snapped Khan's head back and he was soon caught by a crisp left hook.
Khan managed to absorb a right hand but was then caught by a second left hook from Prescott and his legs buckled before he hit the deck.
It looked as though he would struggle to beat the count as his legs threatened to go from under him, but he managed to convince the referee he was fit to continue after taking a full eight count.
But when the bout resumed he immediately walked onto a right hand from Prescott and another right and left from the Colombian was enough to send Khan crashing and ruin his unbeaten record.
Inevitable, I think. Not that he'd lose to this guy specifically. But that he would eventually lose to someone who would take advantage of Khan's poor defence before he could let fly himself.