TOKYO (BLOOMBERG) - Women in Japan are going through an identity crisis.

They're fighting to overturn a law that bars married couples from having different last names, which creates complications for women who have established careers and reputations.

About 600,000 Japanese couples wed every year. The law says that after marriage, a couple must have the same surname. Technically, men may take their wives' family name. Yet in practice, only about 4 per cent do. Some women say they feel like they're wiping away their identity after getting married.

"Being forced to change your name is nothing more than a violation of human rights," said Ms Miki Haga, 29, who is planning to study in the UK this year. She legally became Miki Ishizawa two years ago when her husband didn't want to change his name.

The issue roared into the public debate over the past few weeks during the campaign for the Upper House, where opposition parties have made gender equality a key part of their platform against Prime Minister Shinzo Abe and his ruling Liberal Democratic Party (LDP).

Polling shows the LDP bloc on Sunday (July 21) will grab the majority of open seats. If the opposition makes any major gains, it may try to use gender issues to weaken the LDP's grip on power.

In a striking moment, Mr Abe was the only person on a debate stage earlier this month who didn't raise his hand when asked about support for changing the law. His conservative party argues that the current law is equal to both men and women, and it's a matter of tradition.

"If you believe traditions are important, then there's no need to change the law," said Mr Shigeharu Aoyama, an Upper House LDP member. But others point out that it's not exactly an ancient tradition. Before the current law was passed in 1898, Japanese people didn't typically use surnames. In 1948, it became legal for couples to choose either spouse's surname, but they still had to stick with one.

The surname issue is only one of a number of ways Japan lags behind on gender. Japan has the third-highest gender-pay gap among Organisation for Economic Cooperation and Development countries.

Women are poorly represented in business and politics. They hold only 4 per cent of managerial positions, 2 per cent of seats on boards of directors and about 10 per cent of the seats in the Lower House. The #MeToo movement has had difficulty gaining traction. Although Mr Abe has ginned up support for "Womenomics" - the idea that more women working will help the overall economy - progress has been slow.

A government survey released last year showed 42.5 per cent of adults supported changing the law - about 7 percentage points higher than five years earlier - while 29.3 per cent opposed the move.

The United Nations has pressured Japan to lift the restriction on surnames. It's led to some unusual marital arrangements - even divorces on paper, while couples stay together.

Others choose to live in the equivalent of a domestic partnership. Ms Yuri Koizumi and Mr Hiroshi Tanaka have been living together for 26 years, raising a son without getting married. Ms Koizumi said she couldn't accept changing the name she was born with. "It's not who I am," she said. Meanwhile, Mr Tanaka, a forest science researcher, worried about what would happen to his academic reputation if he no longer used the same name as the one on his published works.

They can't take advantage of the same tax deductions as married couples. Legally, only one of them is allowed to have custody of their son. And they get tired of explaining to new friends and co-workers that they really are husband and wife, and their kids really are theirs, even though they have different last names. The situation is that uncommon in Japanese society.

Courts in Japan recently have upheld the law several times. In 2015, Japan's Supreme Court said the law didn't violate the Constitution. A Tokyo court earlier this year ruled against a similar challenge, and the plaintiffs plan on appealing.

One of those plaintiffs is Mr Yoshihisa Aono, the chief executive officer of software company Cybozu. He legally took his wife's last name when they married in 2001 but continued to use his birth name professionally. His shares are registered under his legal last name - Nishibata - leading to confusion among investors about why the CEO doesn't appear to own a stake in the company. And rules on which name should be on contracts vary by country.

The law has prompted some people to go by their birth names in public, while using their spouse's last name on official documents. That can be tricky. Women worry about whether their academic degrees will be recognised abroad. Companies sometimes mistakenly book flights or hotel rooms for employees under the name they use in everyday life, rather than the legal name they need to use when checking in.

The continued support for the law is based in part on an antiquated Japanese ideal that "individuals are second to the masses", said Mr Toshihiko Noguchi, a lawyer for one of the plaintiffs.

Mr Abe's solution has been to encourage employers to allow workers to informally use the last names they were born with. This November, people will be allowed to list both last names on certain government ID cards, allowing them to open bank accounts or take out loans with their surname of choice.

It's not seamless. Ms Haga gets questioned at airports by border officials who don't understand why both names are listed on the passport. She tweeted her frustration, and a government official responded and pledged to publish an explanation online.

She says every time she filled out another form to legally make the switch - on her bank accounts, passport, credit cards and more - a bit of herself faded. Her husband says he's sympathetic about all the paperwork she had to go through and believes the law should be changed, but he still says he wouldn't have reversed roles.

"My husband didn't have to do anything," Ms Haga said. "It didn't feel fair."