They wedded as man and wife in October 2015. In June last year, the man went for a sex change and updated the identity card to say "female". For eight months, they were a married same-sex couple here - until the marriage was voided in February.

It was one of three voided by the Registry of Marriages (ROM) to date, in a little-known proceeding that has caught family lawyers by surprise. The three unions were voided "for reasons including impersonation by one party", an ROM spokesman told The Straits Times.

He did not give more details.

The case of the same-sex couple was first reported by news site Quartz last month. The pair had booked an HDB Build-to-Order (BTO) flat and proceeded to inform the board last August of the sex change, which meant both were now women.

The HDB then told them to wait. The matter stretched on for months as the authorities decided if the couple were legally married and if they were entitled to a subsidised flat meant for married couples.

In February, they were told that their marriage had been voided.

When asked about the case, the ROM spokesman told The Straits Times it could not share specific details of individual cases, citing privacy concerns, but noted that a marriage here must be between a man and a woman.

HEAR PARTIES OUT I agree in terms of the validity that same-sex couples cannot get married and if they do, it is not recognised. But parties at the very least should be heard. FAMILY LAWYER IVAN CHEONG

"Singapore law does not recognise a marriage where both parties are of the same sex. At the point of marriage, a couple must be man and woman, and must want to be and want to remain as man and woman in the marriage," he added.

If there are reasonable grounds for doubt, the ROM will review the facts and circumstances before deciding if a marriage is valid, he said.

With the marriage voided, the couple lost their BTO flat, reported Quartz. They ended up buying a flat under the Singles Scheme that was half the size of the one they initially booked, and cost $15,000 more as they did not have subsidies available to married couples, it said.

Family lawyers The Straits Times spoke to said that while Singapore laws make clear that there are no same-sex marriages here, they questioned the ROM's authority to void the marriage without going to court. A marriage of convenience, in contrast, has to be determined by a court that it is not a genuine marital relationship.

Family lawyer Sim Bock Eng said: "There is no requirement that parties must remain of the same gender throughout the marriage, nor is there any provision that such marriages (where parties undergo gender reassignment after marriage) are void."

Another family lawyer, Mr Ivan Cheong, said: "I agree in terms of the validity that same-sex couples cannot get married and if they do, it is not recognised. But parties at the very least should be heard."

Transgender people, however, can be married in Singapore - to someone of the opposite sex.

The ROM spokesman said: "According to Singapore law, a marriage is a union between a man and a woman."

There are no official figures for transgender Singaporeans who have married but some, such as Mr Deveshwar Sham, 32, are open about their situations.

The dog handler, who started a support group for transgender men, Kopitiam Brothers, married his wife in October last year after dating for six years.

Mr Sham had his sex reassignment surgery done in June last year. "I think it is a dream come true. It's hard for a relationship to last if you cannot get married," he said.