Ben Simmons has officially ruled out playing for the Boomers at the World Cup, just a day after signing a rich five-year contract extension with the Philadelphia 76ers.

Simmons confirmed his withdrawal in a statement on social media on Wednesday, saying he would forego the World Cup to instead work on his game before the next NBA season.

On Tuesday, the star forward signed a $242-million deal with the 76ers –a record NBA contract for an Australian athlete.

Simmons will still return to Australia for pre-tournament exhibition games against the US and Canada in Perth and Melbourne. He also committed to the Boomers’ 2020 Olympics campaign.

“I wanted to let everyone know that after consulting with my representation, I’ve made the difficult decision to forego playing in the World Cup in China this summer,” Simmons said.

“I’m really excited about the talent we have on the Boomers squad, especially moving closer to 2020, where I will be honoured and humbled to represent my country on the world’s biggest sporting stage at the Olympics in Tokyo.

“Ultimately, we decided it was best that I use the time in September to return to Philadelphia to acquaint myself with my new teammates and prepare for the upcoming NBA season.”

😤🇦🇺 pic.twitter.com/CEmulzcqcR

— Ben Simmons (@BenSimmons25) July 16, 2019

The Boomers are still poised to take their strongest-yet squad to a major event, as they bid for a maiden international medal at the World Cup in China. The squad will feature eight current and former NBA players, including Andrew Bogut, Joe Ingles, Patty Mills, Aron Baynes and Matthew Dellavedova.

Philadelphia is going all-in for the NBA title next season with the extension for Simmons and deals for All-Star big men Joel Embiid and Al Horford and forward Tobias Harris totalling a combined $US580 million ($A827 million).

While 76ers general manager Elton Brand labelled Simmons “one of the NBA’s most dynamic and talented young players”, the Australian’s unwillingness to shoot the ball from outside the key is the glaring weakness in his game and has been exploited by rival NBA teams.

Harris, who agreed to return to the 76ers on a five-year, $US180 million ($A257 million) deal, said he recently worked out with Simmons in Los Angeles and confirmed the Australian was working on shooting from distance and “his jump shot is looking good”.

Harris said Simmons made two consecutive three-pointers over him.

Simmons is yet to make a three-pointer in an NBA game and 90 per cent of his shot attempts come less than three metres from the basket, an incredible statistic for an NBA guard-forward.

“It was a priority for our organisation that we finalise a contract extension with Ben this summer,” Brand said.

“He was Rookie of the Year in his first season, an All-Star in his second and we expect him to grow and succeed for seasons to come.

“Ben positively impacts the game in so many ways and we look forward to continuing our championship pursuits with him as one of our leaders.”

-with AAP