Parliament seats are filled by politicians who win at general elections held every five years or so. But what happens when a seat becomes vacant before a general election is due?
A special election, called a by-election, can be held to fill it. And voters in Bukit Batok SMC, whose MP David Ong resigned on March 12 over an alleged affair, can expect to vote in one soon to elect a new MP.
There is no time limit within which a by-election has to be called and the prime minister has the discretion to determine when it should be held. But Prime Minister Lee Hsien Loong has said he would call one for Bukit Batok "in due course".
The past decade has seen two public debates over whether and when a by-election should be held.
The first was triggered when then Jurong GRC MP Ong Chit Chung - who was overseeing Bukit Batok ward, which was then part of the group representation constituency - died in July 2008.
Then Minister in the Prime Minister's Office Lim Boon Heng said no by-election was required under the law and that Dr Ong's workload would be shared among his fellow Jurong GRC MPs.
Prime Minister Lee Hsien Loong... said Singapore's electoral system focused on political parties, not individuals. Hence, MPs cannot switch parties and still keep their seats. Neither can they resign to force by-elections at will. Furthermore, the mandate of the Government is not affected by the vacancy.
A month later, Nominated MP Thio Li-ann filed a motion in Parliament calling for new laws to require a by-election should the minority member or half or more members of a GRC vacate their seats in midterm.
Responding to set out the government position on the matter, PM Lee said Singapore's electoral system focused on political parties, not individuals.
Hence, MPs cannot switch parties and still keep their seats. Neither can they resign to force by-elections at will. Furthermore, the mandate of the Government is not affected by the vacancy.
Dr Thio's motion was voted out.
The issue surfaced again in February 2012 when opposition MP Yaw Shin Leong lost his Hougang seat after being expelled from his party for refusing to account for an alleged extramarital affair. A by-election was held in May 2012.
Hougang resident Vellama Marie Muthu made a bid to get the courts to declare that the prime minister does not have unfettered discretion to decide whether and when to call by-elections.
In July 2013, the Court of Appeal ruled that the law requires the prime minister to call an election to fill a seat vacated by an elected MP. However, this applies only to single-member constituencies as there is a provision requiring all MPs of a GRC to vacate their seats before an election can be held.
The court also said there is "no pre-determined timeframe" for the prime minister to call a by-election.
"He must do so within a reasonable time and, in that regard, the PM is entitled to take into account all relevant circumstances and only in clear cases can there be judicial intervention," it said.
A by-election was also held in Punggol East in January 2013 to fill the seat vacated by then MP Michael Palmer, who was also Speaker of Parliament.
He resigned after admitting to an affair with a People's Association employee in his constituency.