Barry boxer Lee Selby plans to become a two-weight world champion after failing to retain his IBF World Featherweight title.
Selby was the bookmakers favourite against Leeds' Josh Warrington, at Elland Road. .
Warrington maintained his perfect professional record and ended Selby's near three-year reign as world champion at 126lbs.
Warrington was given the nod by two of the judges - 116-112 and 115-113 - with a third scoring the fight 115-113 in Selby's favour.
Another domestic dust-up may loom for Warrington as the winner of this bout had been heavily linked with Northern Ireland's Carl Frampton, whose next fight has been slated for Windsor Park later this summer.
Frampton - who, like Warrington, is promoted by Warren - was ringside in Leeds to witness the unexpected result.
Selby was expected to make his superior height and reach count against a challenger who received deafening welcome to the ring, with rock band Kaiser Chiefs performing his entrance music live.
Warrington's entourage included former Leeds captain Lucas Radebe and, perhaps buoyed by the atmosphere, he made the quicker start and a couple of left hands seemed to shake the champion.
A head clash in the second led to blood gushing out of the left eye of Selby, who was trying to establish a jab at the outset but a couple of barrages from Warrington brought the crowd to its feet.
The better work was again coming from Warrington into the third as Selby struggled to settle, arguably taken aback by the hostile reception and the early pace of his opponent.
Selby was electing to stand and trade with Warrington into the fourth as he looked to tire the Yorkshireman out by focusing his work to the body.
However, he was unable to hold off Warrington's relentless attacks, the challenger's pressure and flurries bringing him plenty of reward in the fifth and into the sixth.
Another clash of heads opened up a cut over Selby's right eye although it was the left that was more problematic, with the ringside doctor called to assess the extent of the injury.
Given the go-ahead to continue, Selby was once more forced on to the back foot and a crunching left had the Barry fighter seemingly hanging on by the end of the sixth.
Blood was freely flowing out of Selby's left eye, though Warrington's pace had started to slow into the second half of the contest.
Selby started to up his tempo and enjoyed his best rounds in the eighth and ninth although he felt he was being punched in the back of the head by Warrington.
Another right hand from Warrington seemed to take the wind out of Selby in the 10th as he sagged onto the ropes but the pre-fight favourite came bouncing back and ended the round strongly.
There were more furious exchanges in the 11th but neither man was wobbled while Selby arguably shaded the final round of a thriller that will linger long in the memory.