Former All Black Sonny Bill Williams has been stripped of his professional boxing belts after the dual-code rugby player failed to defend his international and New Zealand heavyweight titles.

Williams is eyeing a return to rugby union in 2015 hoping to be part of New Zealand's World Cup defense and also feature in the national ­sevens team at the 2016 Olympic Games in Rio.

His new goals have forced the 28-year-old to suspend his boxing career during which he won all of his six professional heavyweight bouts.

This has prompted the World Boxing Association (WBA) to declare vacant the heavyweight international title, which Williams won in controversial circumstances over South African veteran Francois ­Botha in February.

Botha had Williams on the ropes before the fight was cut by two rounds from the scheduled 12 amid jeers from the crowd. Williams said both boxers knew it would be a 10-round fight.

"He's no longer the WBA international champion," a WBA spokesperson told Fairfax media. "He's been stripped."

New Zealand Professional Boxing Association has also stripped Williams of his heavyweight belt after he failed to respond to challenges.

"We haven't heard a thing back and, personally, that's rude," Lance Revill, the association's president, told NZ Newswire.

"He's not even said he's received the challenge and that they're going to think about it. If that was the case we'd hold off, but we've got no choice now but to strip the title."

A World Cup winner with New Zealand in 2011 and Super Rugby champion with Waikato Chiefs, the controversial Williams opted to stay with the National Rugby League champions ­Sydney Roosters for 2014. His abrupt career changes have enraged fans and officials in both codes and even his decision to play in the recent league World Cup involved a backflip on his original plan and another player being dropped.