The city-state requires all male citizens aged 18 to serve two years in the military, the police or the emergency services, an obligation authorities rarely let people skip.
But Schooling was allowed to defer national service in 2014 to train for the 2016 Rio Olympics, where he famously beat his idol Michael Phelps to win the 100-metre butterfly gold.
He got another deferment to train for the 2020 Tokyo Olympics, but the Games were moved to next year because of the coronavirus pandemic.
Singapore''s defence ministry said Schooling and another Tokyo-bound Singapore swimmer, Quah Zheng Wen, have applied to further postpone their national service to compete in the Olympics.
The ministry said it would examine "details of their training plans and milestones" before making a decision.
Schooling faces a tough fight to repeat his 2016 success -- he was criticised for being out of shape at last year''s Southeast Asian Games in the Philippines, where he won only one individual gold medal.
In 2018, the defence ministry refused an application by Singaporean football player Ben Davis to defer national service so he could play in the English Premier League, saying he did not meet the criteria.
Davis signed a deal with Premier League side Fulham anyway, and could now face jail for missing his national service if he returns to Singapore. (AFP) AT