British boxing champion George Groves was living in fear after gangsters threatened to kill him in a live TV fight.
Police warned Groves, 23, and his fiancee, Sophie, following a tip-off he would be gunned down in the ring.
The British and Commonwealth super-middleweight champ canceled Friday's televised clash with Scottish fighter Kenny Anderson at London's Wembley Arena because of a back injury.
His management said the withdrawal was unrelated to "alleged threats."
But a pal said, "George and Sophie were told by [UK detectives that] a threat had been made on his life and they should consider their safety. The intelligence said if he wasn't shot before the fight, it would happen in the ring."
Anderson had called world No. 12 Groves a "coward" for not fighting him.
But Groves said, "I apologize to everyone who spent money around the fight. I suffered a back injury sparring and haven't trained properly since."
In November 2010, Groves beat Anderson, 29, in the sixth round of a previous bout. It is not known if the rematch will happen.
In 2010, Anderson was cleared of the attempted murder with a hammer of a drug dealer when witnesses failed to turn up at court. There was no sign of Groves or his fiancee at their apartment in Hounslow, west London, on Wednesday.
Police said they are not in protective custody. A spokesman said, "We can confirm a threat to the welfare of a man was made to police in Hounslow."