Japan's failure to recognise same-sex marriage is unconstitutional, a court ruled Wednesday, in a landmark first verdict on the issue that campaigners welcomed as a major victory.

Lawyers for plaintiffs in the case said the ruling should pile pressure on lawmakers to accept same-sex unions, but the path to any such recognition is still likely to be lengthy.

The verdict was the first to emerge from lawsuits filed by more than a dozen couples last year in district courts across Japan, in a coordinated action challenging the only G7 government not to recognise gay unions.

The court in northern Sapporo ruled that the government's current failure to offer same-sex couples ways to "enjoy even a part of the legal effects that arise from marriage... violates article 14" of the constitution, which mandates equality under the law.

Opposition lawmaker Kanako Otsuji, one of the few openly gay Japanese politicians, said in a tweet she was "truly, truly happy" about the verdict.

"With this ruling, I urge the Diet, as the legislative branch of the government, to deliberate a proposed amendment to the civil code to make same-sex marriage possible," she wrote.

The plaintiffs had requested damages of one million yen ($9,000) per person, arguing they were being denied the same legal rights as heterosexual couples.

Japan is the only G7 nation that does not recognise same-sex unions
Japan is the only G7 nation that does not recognise same-sex unions AFP / TORU YAMANAKA

The court rejected the request, saying lawmakers may have struggled to legislate on the issue as the matter has only become a topic of parliamentary debate in recent years.

But the wording of the verdict -- which ruled that sexual orientation is like gender or race and is not chosen at will -- sparked celebrations outside the courthouse.

Lawyers for the plaintiffs, surrounded by rainbow flags, held up a sign declaring the ruling "a huge step towards equality in marriage".

"I was a bit disappointed when I heard the phrase 'turn down' in the verdict, but later I couldn't stop my tears when the presiding judge said it lacks reasonable foundation and is discriminatory," said Ryosuke Kunimi, one of the plaintiffs.

While Japan has some protections for sexual minorities, many same-sex couples struggle to rent apartments together and are even banned from hospital visits.

But the verdict rules that the failure to recognise same-sex marriage violates constitutional requirements for equality before the law
But the verdict rules that the failure to recognise same-sex marriage violates constitutional requirements for equality before the law JIJI PRESS / STR

A November poll by the Yomiuri newspaper found 61 percent of people in Japan are broadly in favour of same-sex marriage, with 37 percent against.

The country's 1947 post-war constitution says that "marriage shall be only with the mutual consent of both sexes".

The government says this means same-sex marriage is "not foreseen" in the constitution or civil law.

But lawyers for the plaintiffs and other legal experts argue the language of the constitution is only meant to ensure equality between prospective spouses and prevent forced marriages.

In a statement, they said they hoped the ruling would prompt parliament to take up the issue.

"We think it is necessary to urge parliament to take swift legal measures by clarifying the current illegal situation, which they have neglected and have not rectified despite their legal duty," they said.

Courts in Tokyo, Osaka and Fukuoka are still considering cases filed by other same-sex couples on the issue.

Lawyers for the Sapporo plaintiffs said they were hopeful that "perfect victories" in those cases would add additional pressure on lawmakers.

Historically, Japan was broadly tolerant of homosexuality, with documented cases of samurai warriors during feudal times having male lovers.

But as the country industrialised and modernised from the late 19th century, Western prejudices were increasingly adopted.

In a landmark advance in 2015, Tokyo's bustling Shibuya district started issuing symbolic "partnership" certificates to same-sex couples.

Some other local governments have followed suit, and corporate Japan is also showing signs of moving towards recognition.

But not all LGBT couples in Japan live in areas with such certificates -- and even those who have them find they are sometimes not recognised.

One of the plaintiffs in the same-sex marriage lawsuits died in January of a suspected brain haemorrhage, and reports said the doctor would not tell his partner what was wrong as he was not a legal family member.

Taiwan is currently the only place in Asia with marriage equality, having taken the unprecedented step of legalising same-sex unions in 2019.