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."