Nick Blackwell is in hospital after suffering a bleed on the brain following his British middleweight title defeat by Chris Eubank Jr in London on Saturday.
The referee stopped the fight in round 10 on the advice of the doctor, who said Blackwell was unable to continue because of swelling over his left eye.
The beaten champion was later taken from the ring on a stretcher.
Blackwell, 25, has been placed in an induced coma.
"It is our understanding that Nick has a bleed on the brain and is under observation by hospital staff," said British Boxing Board of Control general secretary Robert Smith.
On Sunday evening, Smith, who was not at the fight but spoke to Blackwell's camp on Saturday night, added: "There's no change from this morning. He's sedated and staying in overnight. We expect him to be in hospital for some time."
Eubank Jr said: "I hit him with some big shots, maybe the referee should have stopped the fight earlier."
The Brighton challenger had dominated the fight, bloodying Blackwell's nose in rounds three and four with a flurry of jabs and uppercuts.
He was also on top in round five but he was caught with a right after pointing a glove in the direction of Billy Joe Saunders - the only man to beat Eubank Jr - who was at ringside supporting Blackwell.
Eubank Jr continued the onslaught at the start of the following three rounds but each time Blackwell kept his guard high and continued to move forward, catching his opponent with the occasional chopping right.
Round nine was a quieter affair but Eubank Jr used his jab to keep Blackwell at bay and the 10th was also more pedestrian until the referee stepped in and called the doctor over.
Blackwell received medical attention in the ring before being taken away while receiving oxygen.
"This is what boxing needs, two warriors who stand and fight," added Eubank Jr on Channel Five.
"I have respect for anyone who steps in the ring. It's sad that he had to leave on a stretcher. All I wanted was his belt and I've done that. I've achieved my dream of becoming a British champion."