Sonny Bill Williams will return to rugby union in 2015

Sonny Bill Williams

New Zealand dual-code international Sonny Bill Williams has signed a two-year contract to return to rugby union in 2015 in time for the World Cup.

The Sydney Roosters player, 28, has agreed the deal with New Zealand Rugby Union which will allow him to play sevens at the 2016 Olympics in Rio.

He will play at club level for the Hamilton-based Super Rugby side Chiefs.

"We are delighted to have agreed terms which will see him return in 2015," said NZRU chief executive Steve Tew.

"Sonny has shown he is an outstanding talent and a real asset."

Williams would have to displace either Ma'a Nonu or Conrad Smith for a starting berth for the All Blacks.

Coach Steve Hansen, though, has admitted the centre would be "very hard to leave out" of his side's world title defence in England in September 2015.

Williams, who was this week stripped of his international and New Zealand heavyweight boxing titles, was lured to union in 2008 by French club side Toulon, before returning to his native New Zealand in 2010 to play with Super 15 side the Crusaders.

He made his All Blacks debut in the 26-16 win over England in November of that year and has won another 18 caps, including an appearance as a replacement in the World Cup final win over France in 2011.

A stint in Japanese club rugby preceded his return to league for the 2013 NRL season with the Sydney Roosters. He was a late addition to the Kiwi squad for their Rugby League World Cup defence in England and Wales, having initially said he was unavailable.