Japan's refusal to recognise same-sex marriage has been ruled unconstitutional by a high court, in the latest of a string of decisions examining the ban.

In Friday's ruling by the Fukuoka High Court, presiding judge Takeshi Okada noted that the current civil law provisions barring the marriage of same-sex couples violated their fundamental right to the pursuit of happiness guaranteed in the Japanese Constitution.

The court also said the ongoing ban violated sections in the constitution that guaranteed equality, individual dignity and the essential equality of both sexes.

The judge said there was no longer any reason to not legally recognise same-sex marriage.

Japan is the only member of the Group of Seven (G7) industrialised countries that does not recognise same-sex marriage or provide any other form of legally binding protection for LGBT couples.

The decision is the eighth of nine rulings to find the ongoing ban to be unconstitutional or nearly so, against only one district court decision that found it constitutional.

It was the third-straight high court decision, following rulings this year in Sapporo and Tokyo that also called the current ban unconstitutional.

Pressure on Japanese government

The Japanese government has argued that marriage under civil law does not cover same-sex couples and places importance on natural reproduction.

After the ruling, Chief Cabinet Secretary Yoshimasa Hayashi told reporters that the government would monitor pending lawsuits and public opinion.

The rulings can still be appealed to the Supreme Court, but lawyers and plaintiffs say the overwhelming 8-1 wins are already enough and the government should quickly take action.

Friday's verdict comes as the main obstacle to recognition, Japan's conservative Liberal Democratic Party, has been forced into leading a minority government after its major election loss in October.

The LDP is likely to have to compromise more on liberal policies pushed by the opposition parties such as marriage equality, which is largely supported by the general public.

Japanese PM will not 'sit back'

Prime Minister Shigeru Ishiba, considered a centrist who has indicated support for same-sex marriage, told a parliamentary session last week he was aware of the challenges facing same-sex couples and that he would not just "sit back and do nothing."

Mr Ishiba's comment raised hope among the LGBT community and their supporters.

It is unclear whether he will act, or how much he could do against resistance within the party.

Two more high court rulings on the constitutionality of same-sex marriage are pending in Osaka and Nagoya, with decisions expected in March.

Takako Uesugi, a lawyer representing plaintiffs in Tokyo, said that with all three high court rulings in favour of their campaign, she has high expectations for the two upcoming decisions.

"We'll continue fighting until we achieve legalisation of same-sex marriage," Ms Uesugi said.

AP