Lee Selby plans to step up to junior lightweight after Saturday's split points defeat to Josh Warrington at Elland Road.
The Welshman lost the IBF world featherweight title to Warrington, who most observers thought deserved to win on all three scorecards, in front of over 20,000 in his home city of Leeds.
It was Selby's first defeat for nine years and ended his three-year reign as world champion after five successful defences, also costing him a lucrative clash with Northern Ireland's Carl Frampton at Windsor Park, Belfast, on August 18.
But in a statement through social media, Selby revealed he has been struggling to make the featherweight weight limit of nine stones.
"I was Britain's longest reigning world champion but it came at a price and I felt that in the ring," Selby said.
"I've been making the featherweight limit for ten years but it was just one fight too many at that weight and my performance badly reflected that.
"Congratulations to Josh Warrington and I wish him the best in the future.
"I will enjoy the summer with my family and then sit down with my team to plan the exciting new challenge of becoming a two-weight world champion."
Warrington (27-0, 6 KOs), 27, is not expected to fight Frampton this summer and instead is going to be more busy with parenting duties after his wife Natasha gave birth to twin girls 12 weeks ago.
But Warrington hopes winning his first world title will boost his profile in the U.K. and dreams of fighting in Las Vegas.
"I don't think I'm going to become a national hero overnight but people might start giving me a bit more credit for what I have achieved," Warrington said.
"They said when Ricky [Hatton] went over [to Las Vegas] it was barmy, they will get the shock of their lives if Leeds fans went over.
"I always said the plan was to win a world title, defend it, have a couple of away days, possibly go to Vegas and knock it on the head. But we'll see what happens."