TOKYO -- A Tokyo court rejected on Thursday a lawsuit by Japanese living in Europe who sought to retain their nationality even after taking foreign citizenship, instead of losing it automatically, as happens now.

Japan has been negative about dual citizenship, requiring citizens with more than a single nationality to choose one, even though more nations in recent decades have allowed dual citizenship, or turned a blind eye to it.

“I had to lose my Japanese citizenship just before I went to study in university in Japan,” said one of the plaintiffs, Yuki Shiraishi, who had lived in Switzerland, where her parents had migrated.

“I felt a big sense of loss when I saw holes on my Japanese passport,” added Shiraishi, who joined the suit hoping Japan would grant dual citizenship.

The lawyers pursuing the case said the plaintiffs plan to appeal against Thursday’s decision.

The government has said in the past that dual nationality could complicate matters of taxation and diplomatic protection and is not in line with the concept of a sovereign state.

But about 900,000 people are estimated to hold dual citizenship in Japan today, including naturalised citizens and the offspring of marriages contracted with foreigners.

Although required by law to renounce their foreign nationality or declare that they will, Japan has never enforced the rule, and legal ambiguity forces many of them to keep silent.

Conservatives in Japan are largely opposed to dual citizenship, however.

“First of all, why do you even want to have dual nationality?” asked one Twitter user. Another wrote it was selfish for people to want a foreign passport while holding on to Japanese citizenship.

Against this backdrop, the plaintiff’s lawyers have taken care to target a part of the law that forces Japanese to lose citizenship, rather than directly tackling the issue of dual nationality.

The plaintiffs say the unfair treatment makes the law unconstitutional, by forcing only Japanese nationals gaining foreign citizenship abroad to follow the rule on dual nationality, while ambiguity cloaks those at home.

Apart from infringing individual rights, they said, the current law hurt the national interest by causing the loss of talented people who live abroad, especially as ageing shrinks the domestic population rapidly.

“It’s not reflective of how international the world has become – it’s holding Japan back in a big way,” said Hannah Ogahara, a British national living in Tokyo, the capital, who gave up Japanese citizenship a few years ago.

She said she believed the government wanted to keep a clear demarcation of those with Japanese nationality, rather than blurring the line by allowing dual citizenship.