Billy Joe Saunders extended his professional record to 18-0 and inflicted a first defeat on Gary O'Sullivan with a unanimous decision at Wembley Arena on Friday.
Irishman O'Sullivan came into the bout on a 16-fight winning run and tried to force the pace in the early rounds.
But Saunders - who turned pro after representing Great Britain at the 2008 Olympics - became increasingly dominant in the middle and later stages of the 12-round contest to earn a lopsided verdict on the scorecards and add the WBO International middleweight strap to his collection.
Two judges scored it 120-109 in favour of Saunders and the other gave it to him by 119-110.
Saunders' next assignment is a defence of his British middleweight title against John Ryder at the Copper Box Arena in London on September 21.