Terri Harper beat Rhiannon Dixon by unanimous decision to become a three-weight world champion.
The 27-year-old, who had previously held world titles at super-welterweight and super-featherweight, claimed the WBO world lightweight belt with scores of 96-94, 97-93, 97-93.
"That's the best one of my career. I've become a new fighter mentally and physically," Harper - the light-middleweight and former super-featherweight champion - said.
"There was one point in that fight where I was clocked and I really had to dig deep, but I managed to recover quite quick."
Dixon was defending the world title for the first time following her win over Karen Elizabeth Carabajal in April.
'Truth will be set free' says Ryan as Mayer denies paint-throwing claims
Harper's former opponent, Sandy Ryan, was hit with a can of red paint before losing her WBO welterweight world title to Mikaela Mayer in New York as she pointed the finger at her rival over the incident.
Tensions had been high prior to the fight, with Mayer accusing Ryan of poaching her long-standing coaching team, which Ryan denied. Ryan had said she did not respect her opponent, accusing her of talking "trash".
But the spiteful rivalry escalated when Ryan was splashed with red paint ahead of the fight, before claiming she had been "set up".
Ryan took the ring at Madison Square Garden as scheduled, despite being advised by her trainer to pull out of the bout, but lost on a majority decision, leaving Mayer as the world champion.