People's Action Party MP for Bukit Batok David Ong resigned as both an MP and party member yesterday after admitting to a "personal indiscretion".
Mr Ong, 54, is believed to have had an extramarital affair with a married woman, who is a grassroots activist in his constituency.
Prime Minister Lee Hsien Loong said a by-election will be called "in due course" in Bukit Batok, which the PAP won with 73 per cent of the votes in a three-way race in last September's general election.
The Singapore Democratic Party (SDP) said it will be contesting the seat and will hold a walkabout in the area this morning.
Mr Ong, who is married with three teenage children, told The Sunday Times: "I feel that it's the right thing to do to step down for the best interests of the party, my constituents and family. There is personal indiscretion on my part, which I deeply regret."
He added: "I would appreciate if you can give me and my family the privacy to heal and rebuild."
He is the third MP in five years to resign from his seat because of an affair. The Sunday Times has learnt that the woman in question is Ms Wendy Lim, 41, a PAP member who works in a logistics firm.
Yesterday, many residents and volunteers said they were surprised at Mr Ong's sudden resignation, adding that they appreciated his hard work in the constituency and were sorry to see him go.
In addition to the SDP, which contested the seat at the Sept 11 general election, several other opposition parties could also stand in the by-election.
It would be the third by-election following Hougang in 2012 and Punggol East in 2013.
PAP leaders assured Bukit Batok residents that their needs would be attended to in the meanwhile.
PM Lee said Mr Ong "has worked hard as an MP and served his residents diligently", and Senior Minister of State for Home Affairs and National Development Desmond Lee, an MP for neighbouring Jurong GRC, would take care of residents in the meantime.
Deputy Prime Minister Tharman Shanmugaratnam also told reporters the party's first priority is taking care of Mr Ong's constituents.
"I want to say to Bukit Batok residents, on behalf of the party, that we are deeply sorry this has happened," said Mr Tharman.
Although it is a single seat, Bukit Batok is "part of the Jurong family", and Mr Lee will take over Mr Ong's Meet-the-People sessions. Jurong GRC MP Ang Wei Neng will take over Mr Ong's role as Jurong-Clementi Town Council chairman.
Asked when the PAP found out about the matter, Mr Tharman said it was "totally unexpected, very sudden". As for whether there had been rumours before the general election, he said: "Certainly not."
Mr Ong is managing director of a publishing company. He was a long-time PAP and grassroots volunteer in Kreta Ayer-Kim Seng when he was asked to contest the 2011 general election, where he was among the oldest of the new faces.
He was elected MP for Jurong GRC, and his Bukit Batok ward was made a single seat for GE 2015.
In his letter to PM Lee, Mr Ong said he was resigning "for personal reasons, and with a heavy heart". He said he was grateful to his residents and apologised for having to step down prematurely.
PM Lee noted Mr Ong's diligent service, his parliamentary speeches on issues such as the retirement age, and his other community roles.
"However, having considered the matter carefully, I believe that your decision to resign is in the best interest of your constituents, the party, your family and yourself," he wrote.
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