The Court of Appeal has ruled that the Prime Minister must call an election to fill a seat vacated by an elected Member of Parliament, overturning a previous decision by a High Court judge that the PM has the discretion to decide whether or not to call an election.

The decision brings to an end a long-running court case which has sparked much discussion since Hougang resident Vellama Marie Muthu applied for the High Court to rule on the issue in March last year after former Hougang MP Yaw Shin Leong vacated his seat.

Last August, Justice Philip Pillai dismissed Madam Vellama's bid after noting that the case turned upon the meaning of one word in Article 49 of the Constitution, which says a vacancy "shall be filled by election". Madam Vellama's lawyer M. Ravi had argued that the word "election" meant an election had to be held, while the Attorney-General's Chambers said "election" described the process of how a vacancy is to be filled. The judge stood with the latter interpretation.

Madam Vellama, a cleaner, had appealed against Justice Pillai's ruling. But after reserving judgement in January this year, the Court of Appeal said in its written judgement on Friday that the key lay in the word "shall" rather than "election". "Shall" indicates the act is mandatory, said the apex court, repeating an argument that Mr Ravi had earlier made.

While a previous clause saying that an election shall be called in three months had been deleted in 1965, the Appeals Court said: "The absence of the time-limit clause cannot lead to the conclusion that the Prime Minister is completely free to do as he pleases, even to the extent of delaying indefinitely the calling of a by-election or even declaring that he will not fill the casual vacancy."

On the other hand, the court agreed with the AGC that there is "no pre-determined time frame" for the PM to call a by-election but noted that a vacancy left unfilled for "an unnecessarily prolonged period" raises the serious risk of disenfranchising constituents. "He (the PM) must do so within a reasonable time and in that regard, the Prime Minister is entitled to take into account all relevant circumstances and only in clear cases can there be judicial intervention," said the court.

The court said it was dismissing Madam Vellama's appeal as she did not have standing to seek relief after the Hougang by-election was held on May 26 last year. But due to the circumstances and as the court was largely with Madam Vellama on her interpretation of Article 49, it ordered each party to bear their own costs.