A by-election will be held in Bukit Batok parliament constituency as early as in May, reports have said.
Following David Ong Mp's resignation, Prime Minister Lee Hsien Loong had said a by-election will be held in due course.
Ong resigned on Friday as MP and quit the ruling People's Action Party as well, after details of his extra marital affair with a fellow PAP member emerged in public.
"On 12 March 2016, I tendered my resignation as a Member of Parliament to the Speaker. I hereby also tender my resignation as a member of the People's Action Party with immediate effect. I am doing so for personal reasons and with a heavy heart," Ong said in his resignation letter.
Local media reported that Ong said in a personal statement that he admitted to "personal indiscretion on my part which I deeply regret."
"I feel that it's the right thing to do to step down for the best interests of the party, my constituents and family....I would appreciate if you can give me and my family the privacy to heal and rebuild," wrote Ong, a married father of three children.
The woman is believed to be 41-year-old Wendy Lim, grassroots PAP member and a logistics firm employee, according to t he New Paper. Chinese daily Lianhe Zaobao revealed the affair, saying the husband of the woman allegedly lodged a complaint.
Prime Minister Lee said he deeply regretted that he had to accept Ong's resignation.
Lee also said Desmond Lee, Senior Minister of State for Home Affairs and National Development, will represent the people of Bukit Batok until a by-election is held.
Some observers believe the by-election will be held immediately after Parliament completes its debate on the Budget on April 15. However, The Straits Times reported, citing experts, the government may adopt a more cautious line as it expects the fight to be tough this time.
Though Ong won 73 per cent of the vote in the last general election, observers think the by-election will be a closer fight, the daily said.