Six couples sued the Japanese government on Friday seeking the right to use different surnames after marriage, filing the case to coincide with International Women's Day.

Under laws in place since the 19th century, married couples must choose the husband's or the wife's name, and about 95 percent take the man's, according to the plantiffs' lawyers.

Apart from the bureaucratic headache of having to change names on everything from passports to bank accounts, this creates problems for women with established careers, campaigners say.

And if couples don't marry this affects a host of rights including those surrounding children, inheritance and tax.

One of the plaintiffs, a 50-year-old who declined to be named, said she and her partner have lived together for 17 years and have brought up a teenage daughter, but without marrying.

"Both of us hated the idea of changing our surnames but didn't force the other to change name," the woman told reporters.

"But the fact that we are not legally a married couple -- even though we've lived together as a family for 17 years -- could cause problems such as being unable to become a legal heir, or being unable to provide consent for a surgery, or tax law-related disadvantages," she said.

Other plaintiffs, Yukio Koike, 66, and his partner Yukari Uchiyama, 56, got married every time their three children were born in order to have joint custody rights -- and then divorced.

It is "because we want to respect the personality of each other," Koike said. "I've never thought of erasing my name in my life."

Five of the couples filed their case at the Tokyo District Court and the sixth in Sapporo in northern Japan.

The suit seeks "confirmation of illegality of the government's failure to amend the law" and compensation of 500,000 yen ($3,400) per plaintiff, according to a statement.

"In countries outside Japan, surname and marriage... are not in a trade-off relationship. But in Japan, if you choose one you have to give up the other," lead lawyer Makiko Terahara told reporters.

Japan's Supreme Court has twice, in 2015 and in 2021, ruled that the current law is constitutional but it also urged lawmakers to discuss a bill addressing growing calls for flexibility.

Calls to allow separate surnames have been growing in recent years.

Last month, Masakazu Tokura, head of Japan's most influential business lobby Keidanren, said the group supports introduction of the option when getting married.

Backers of the current laws say having a single family name is important to promote family ties and that efforts to change the rules are an attack on traditional values.