A Japanese court ruled on Monday that the country's failure to recognise same-sex marriage is constitutional, in a setback for activists after a landmark verdict last year found the opposite.

The district court in western Japan's Osaka rejected arguments made by three same-sex couples as part of a series of suits filed by activists seeking marriage equality.

"From the perspective of individual dignity, it can be said that it is necessary to realise the benefits of same-sex couples being publicly recognised through official recognition," the court ruling said.

But the present failure to recognise such unions is "not considered to violate... the Constitution", the ruling added, saying "public debate on what kind of system is appropriate for this has not been thoroughly carried out".

The verdict comes after a district court in northern Sapporo last year found the opposite, ruling that the government's failure to allow same-sex marriage violated the constitution's provision guaranteeing equality under the law.

That ruling was welcomed by campaigners as a major victory that would pile pressure on lawmakers to accept same-sex unions.

Japan's constitution stipulates that "marriage shall be only with the mutual consent of both sexes".

But in recent years, local authorities across the country have made moves to recognise same-sex partnerships, although such recognition does not carry the same rights as marriage under the law.

The prefecture of Tokyo last month said it would begin recognising same-sex partnerships from November, revising current rules.

More than a dozen couples filed suits seeking marriage equality in 2020 in district courts across Japan. They said the coordinated action was intended to put pressure on the only G7 government that does not recognise gay unions.