Rio 2016 gold medallist Chloe Esposito is vowing to return to the sport ©Getty Images

Australia's reigning Olympic Games modern pentathlon champion Chloe Esposito is pregnant and will not be defending her title at Tokyo 2020.

The Games begin on July 24, with Esposito due to give birth to her first child in August.

"A wonderful unexpected surprise has happened," Esposito tweeted.

"My husband, Matt Cooper and I are bringing a mini Cooper into this world in August.

"True to form, nothing ever works out the way we plan it.

"Defending my title will have to wait another four years.

"Can't wait to be a mum."

The 28-year-old Esposito is not planning to retire, after becoming Australia's first Olympic modern pentathlon gold medallist at Rio 2016 and then taking a year off.

However, she completed an impressive return to action, winning the World Cup final in 2018 and becoming world number one again.

While hamstring injuries curtailed her performances last year, Esposito was looking to compete prior to the news of her pregnancy.

"I would like to express my congratulations to Chloe and Matt… they will be fantastic parents," Modern Pentathlon Australia President Kitty Chiller said.

"We would have of course loved to have seen Chloe on the start line in Tokyo to defend her Olympic title.

"However, we know this is not the end of her remarkable career and look forward to supporting her in due course when she returns to the sport with an eye to the Paris Olympics in 2024."