BANGKOK - Thai lawmakers will vote on June 18 to legalise same-sex marriage, putting the kingdom on the cusp of becoming the first South-east Asian nation to recognise marriage equality.

The senate upper house is expected to approve the legislation, after which it will go to King Maha Vajiralongkorn for royal assent and come into force 120 days after publication in the official Royal Gazette.

Thailand would become only the third place in Asia where same-sex couples can tie the knot, after Taiwan and Nepal, and activists are hoping the first weddings could be celebrated as early as October.

“I am confident that the Bill will pass,” lesbian, gay, bisexual, transgender and queer (LGBTQ) activist Siritata Ninlapruek told AFP.

“I believe that parliament members will realise the importance of equal rights and the family institution. This topic is too big to fail.”

Senators will meet from 9.30am (10.30am Singapore time), with a final vote expected in the afternoon.

The new legislation changes references to “men”, “women”, “husbands” and “wives” in marriage laws to gender-neutral terms.

It also gives same-sex couples the same rights as heterosexual ones when it comes to adoption and inheritance.

Prime Minister Srettha Thavisin, who has been vocal in his support for the LGBTQ community and the Bill, will open his official residence to activists and supporters for celebrations after the vote on Tuesday.

Activists will later hold a rally, featuring a drag show, in central Bangkok, where giant shopping malls have been flying the rainbow flag in a show of support