TOKYO (REUTERS) - Instead of spending the days before her wedding happily planning for the big day, Ms Kaori Oguni was agonising over the prospect of losing her maiden name and with it, she felt, part of her identity.

Ms Oguni is one of five people suing the government of Japan, the only country in the Group of Seven with a law requiring spouses to adopt the same surname.

The women say the law is unconstitutional and violates married couples' civil rights, and are demanding compensation.

"By losing your surname ... you're being made light of, you're not respected ... It's as if part of your self vanishes,"said the 41-year-old translator.

A decision by the Supreme Court, due on Dec 16, coincides with Prime Minister Shinzo Abe's push to draw more women into a shrinking workforce. Despite that, many in his conservative ruling party are opposed to any legal change.

An 1896 law says spouses must adopt the same surname to legally register their marriage. The law does not specify which one, but in practice, 96 per cent of women take their husband's name, a reflection of Japan's male-dominated society.

Conservatives say allowing couples to choose whether they share the same surname or not could damage family ties and threaten society.

"Names are the best way to bind families," Mr Masaomi Takanori, a constitutional scholar, told NHK public television. "Allowing different surnames risks destroying social stability, the maintenance of public order and the basis for social welfare."

Others say it is time for a change. "The world is more oriented towards individuals now," said Mr Shunsuke Serizawa, a social commentator on gender and family roles. "Separate surnames is a natural extension."

Many working women face the hassle of juggling two names - their maiden name for professional use and their legal, married name, required on official documents.

"If changing surnames is so easy, why don't more men do it?" said Ms Oguni. "The system is one that says, basically, 'if you're not willing to change, you shouldn't be getting married'."

Some couples opt not to register their marriage so they can keep separate names, an option taken by opposition Social Democratic Party lawmaker Mizuho Fukushima and her partner.

Doing so, however, creates legal headaches including complications over parental and inheritance rights. Ms Oguni took her husband's name legally but uses her maiden name professionally.

Two previous courts ruled against the women.

Public opinion is divided. A poll by the liberal Asahi Shimbun newspaper last month found 52 per cent in favour of being able to choose and 34 per cent against. Support for the option of separate surnames is much higher among younger people.

Ms Kyoko Tsukamoto, another plaintiff who goes by her maiden name, has been with her husband since 1960 and married him when their first child was born so the child would be legitimate.

They then divorced, and remarried to have another child but her husband refused a second divorce.

"I was born Tsukamoto, and I want to die Tsukamoto," the 80-year-old said.