The Court of Appeal today heard an appeal against a judge's ruling on the Hougang by-election case.

At the end of it, it ordered lawyers for the Attorney-General's Chambers and a Hougang resident to dig into Singapore's legislative history in 1963.

This might shed more light and fill any gaps in the earlier High Court decision on the case, the Appeals Court said.

Both parties agreed to take a month to make further written submissions.

In March last year, Hougang resident Vellama Marie Muthu tried to get the courts to declare that the Prime Minister does not have "unfettered discretion" in deciding whether and when to call a by-election.

This came after the Workers' Party MP for Hougang vacated his seat.

Madam Vellama's case was heard on July 16. Meanwhile, Prime Minister Lee Hsien Loong called a by-election in Hougang on May 9 to fill the vacancy.

Hearing her case, Justice Philip Pillai ruled last year that it is for the PM to decide whether or not to call an election to fill a seat vacated by an elected MP.

The PM also has discretion to decide when to call such an election, he said. The case then went to the Court of Appeal.

Madam Vellama's lawyer M. Ravi said Monday morning that even though the by-election was held, Madam Vellama still has a private right to know if the PM has "unfettered discretion", and if the judge had made his decision erroneously. Public interest was also at stake, he said.

The AGC, represented by Senior Counsel David Chong, argued that the appeal should not be heard in the first place.

The Appeals Court said the AGC's argument does not preclude the court from making a judgment on whether Justice Pillai had erred in his interpretation of law, if he did.

It then ordered both parties to research Singapore's 1963 legislative history.