Nick Blackwell has been placed in an induced coma after suffering bleeding to the brain following his defeat to Chris Eubank Jr on Saturday.
The British middleweight title bout was stopped in the 10th round after the ringside doctor advised the referee Blackwell was unable to see through his left eye.
Blackwell was later carried out on a stretcher and taken to hospital, where the 25-year-old remained under observation on Sunday morning.
British Boxing Board of Control general secretary Robert Smith said: "I spoke to one of the doctors this morning and he said there is a bleeding of the brain.
"He's in intensive care, he's in an induced coma and he is resting and being looked after by the experts.
"It's a very normal procedure. They put you in a coma to get the swelling to go down. There's no time-scale. Now it's just a matter of waiting and seeing."
A source close to the Blackwell team told Press Association Sport: "He's stable and there was no swelling overnight."
Eubank Jr dominated the fight at Wembley Arena and had earlier bloodied Blackwell's nose in rounds three and four, before landing a number of heavy blows in round seven.
World heavyweight champion Tyson Fury sent his best wishes to Blackwell on Saturday night. "will u all pray for @nickblackwell02 please ASAP as he has been taken to hospital, God be with him," Fury wrote on Twitter.
Former world cruiserweight champion Tony Bellew wrote: "Woke up this morning an the first person on my mind was @nickblackwell02. I just pray he comes out this in the best way possible."
Former super-middleweight champion George Groves has spent time sparring with Blackwell in recent weeks. He wrote: "Don't like thought of a tweet being helpful in a time like this but thoughts and prayers of myself and family are with @nickblackwell02. I hope he comes out of this ok. We've been in the gym sparring together the past month and he's a truly one of the sports great guys."
Former world heavyweight champion Lennox Lewis added his own get-well message: "Thoughts and prayers to @nickblackwell02 and his family at this time! #BlackwellEubankJr".
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