Schooling confessed to using cannabis while on a break from national service to participate at the Southeast Asian (SEA) Games in Hanoi in May, despite the drug not being flagged in a drugs test.

Schooling won two gold medals and a bronze medal at the SEA Games.

Both Schooling and fellow swimmer Amanda Lim were investigated by the Central Narcotics Bureau (CNB) over alleged drug use, as revealed by national sports body Sport Singapore.

As he is serving a two-year period mandatory national service - which began in January this year - Schooling was referred to the Ministry of Defence, which ruled he was no longer eligible for leave to train or compete for the rest of his service.

It means Schooling is set to miss the SEA Games in Cambodia next year, as well as the Hangzhou 2022 Asian Games, postponed to 2023 because of COVID-19 restrictions in China.

Schooling's national service is expected to end in January 2024, leaving him little more than six months to prepare for the Paris 2024 Olympics.

Lim - who was second in the women's 50 metres freestyle in Hanoi and won a gold medal in the women's 4x100m freestyle relay, was issued with a warning by the CNB.

Schooling was also issued a formal warning and is required to undergo supervised urine testing as part of a rehabilitation process.

"I am sorry that my actions have caused hurt to everyone around me, especially to my family and the young fans who look up yo me," Schooling said on social media.

"I gave in to a moment of weakness after going through a very tough period of my life.

"I demonstrated bad judgement and I am sorry.

"I made a mistake and I'm responsible for what I've done."

Schooling's father died in November.

Singapore's Armed Forces has a zero-tolerance policy toward drug use.

Sport Singapore said it will review the incidents as both swimmers currently receiving funding from the body.

"Drugs have no place in our society and we take a zero-tolerance stance towards illegal drug use," said Singapore Swimming Association President Mark Chay, with the national governing body also reviewing its next steps.

"This message, along with the expectations for our national athletes to uphold the highest standards of conduct, will be strongly reinforced amongst our community through our national coaches and affiliates."

Cannabis use is illegal in both Singapore and Vietnam.

People in possession of cannabis can face up to 10 years in prison in Singapore, while drug trafficking is punishable by death.

Half an ounce of cannabis is regarded as presumption of trafficking, an amount that is available at dispensaries in countries where the substance is legal.

In beating American swimming legend Michael Phelps in the men's 100m butterfly final at Rio 2016, Schooling became the first Olympic champion from Singapore, also breaking the national record in a time of 50.39sec.