HONG KONG (Reuters) - A Hong Kong appeals court on Monday ruled that a British lesbian whose partner works in the city should be granted a spousal visa, a landmark decision that could lead to expatriate same-sex partners moving to the Chinese-ruled financial hub.

The British national, known as QT, had been refused a dependant's visa on the grounds that she was not a "spouse", even though she had entered a civil partnership in England.

Marriage in Hong Kong is defined as a monogamous union between a man and a woman, but discrimination based on sexual orientation is against the law. The former British colony returned to Chinese rule with wide-ranging autonomy, including a free judiciary, in 1997.

In a unanimous decision reached by three judges, the Court of Appeal found that the director of immigration failed to justify the "indirect discrimination on account of sexual orientation that QT suffers".

While the legal definition of marriage was not challenged in the appeal, chief judge Andrew Cheung wrote that "times have changed and an increasing number of people are no longer prepared to accept the status quo without critical thought".

"Excluding a foreign worker's lawfully married (albeit same-sex) spouse or civil partner ... to join the worker is, quite obviously, counter-productive to attracting the worker to come to or remain in Hong Kong to work in the first place," he added.

The court ordered QT and the Department of Immigration to work together on an agreement and submit it to court within 28 days.

Hong Kong's LGBT advocates welcomed the decision, with the city's first openly gay lawmaker, Ray Chan, calling it a "big win for same-sex dependants to apply for a visa to stay in Hong Kong".

Same-sex marriage was legalized in Taiwan in May, a first in Asia. In mainland China, homosexuality was listed as a mental disorder until 2001, but it is not illegal to be gay.

(Reporting by Venus Wu; Editing by Nick Macfie)