Amir Khan retained his WBA light-welterweight title with a controversial points decision over Paul McCloskey in Manchester's MEN Arena on Saturday.
A clash of heads in the sixth resulted in a cut above McCloskey's eye and after consultation with the ringside doctor the referee stopped the fight.
All three judges scored the scrappy fight 60-54 in Khan's favour.
McCloskey's furious manager Barry Hearn vowed to lodge a complaint over the stoppage which he called a "disgrace".
Khan's last fight was a stunning points victory over hard-hitting Argentine Marcos Maidana, but this was a scrappy, stop-start affair that never came close to the same sensational standard.
That contest in Las Vegas in 2010 was a fight of the year contender and will live long in the memory but Saturday's meeting with McCloskey will be quickly forgotten.
It was a disappointing encounter, that had no meaningful action, which justified the lack of pre-fight interest from broadcasters, with Khan made to miss far too often as the 24-year-old looked one-dimensional in his approach to defeating 31-year-old McCloskey.
Once the early barrage had failed to break the defence and spirit of the challenger, Khan seemed short of ideas with McCloskey also showing he possessed an iron chin.
McCloskey offered little in the way of attack other than the occasional wild left hook but proved an awkward opponent for Khan who never really troubled the Northern Irishman until the pace slowed considerably at the start of the fifth.
Khan dug McCloskey with left hooks as the challenger continued to load up left hands, only to be caught with a hard right to the chin that wobbled him for the first time.
That buoyed Khan who sprung into life in the sixth and after a trademark flurry of punches a clash of heads left McCloskey with an at first innocuous cut above his left eye.
The fight was stopped for what many believed was going to be a routine inspection, but after a conversation with the ringside doctor the contest was ended.
At first the previously unbeaten McCloskey offered little in the way of a protest, perhaps sensing the tide in the fight was turning, although when interviewed afterwards he toed the party line that he had in fact been cheated.
Hearn was more vigorous in his protests, arguing that McCloskey's corner were never given the opportunity to work on the cut as emotions came close to boiling over inside the ring against a chorus of boos inside the MEN Arena that had hosted around 18,000 spectators.
McCloskey's fans and team were fuming and Khan, while delighted, clearly felt for his opponent.
"I'm sorry it had to end that way," he told McCloskey before shaking hands with the loser.
McCloskey stopped short of accusing Khan of dirty tactics - indeed, the champion seemed blameless - but the Ulsterman was clearly unhappy.
"The way the doctor put it to me, I thought I was badly cut," he said. "He told me it was really bad."
Asked whether he thought Khan was being careless with his head, McCloskey said: "How did I get the cut? I wouldn't say he did it intentionally but it was a clash of heads."
Hearn, who has been a boxing promoter since the 1980s, added: "It is a shameful decision, we will make a complaint and we want a re-match. I have been in the game a long time and I am absolutely astonished, I cannot believe that fight was stopped."
Either way it seemed before the clash of heads that the fight was going only one way and the judges agreed, unanimously awarding the champion victory.
At a chaotic post-fight press conference Khan's side rejected the prospect of a rematch, though McCloskey's co-promoter Eddie Hearn claimed the British Boxing Board of Control have promised to open an investigation.
Khan, though, was bullish.
"He didn't win one round," he said. "I was hurting him, I wasn't tired, and I promise you if it had gone two more rounds he would have been knocked out. There is no point in a rematch."
Khan, who now has a record of 25 victories and one defeat, hopes that a unification fight with WBC and WBO champion Timothy Bradley in the summer will be next. However, after this performance, perhaps his trainer Freddie Roach will look to delay that meeting until later in the year.
Meanwhile, on the undercard, Leicester binman Rendall Munroe made a winning return following his unsuccessful world title challenge last year with a unanimous decision over Andrei Isaeu.
Craig Watson lost his British welterweight title to Lee Purdy after being dropped and stopped in round five.