June 27 (Reuters) - British authorities must reconsider whether to open a probe into the importation of cotton allegedly produced by slave labor in the Chinese region of Xinjiang, a London court ruled on Thursday, allowing an appeal by a Uyghur rights group.

The World Uyghur Congress, an international organization of exiled Uyghur groups, took legal action against Britain's National Crime Agency (NCA) after it declined to begin a criminal investigation.

Rights groups and the U.S. government accuse China of widespread abuses of Uyghurs and other Muslim minorities in the western region of Xinjiang, from where the vast majority of Chinese-produced cotton emanates.

Beijing vigorously denies any abuses and its embassy in Washington has previously described allegations of forced labor as "nothing but a lie concocted by the U.S. side in an attempt to wantonly suppress Chinese enterprises."

The Chinese embassy in London did not immediately respond to a request for comment on Thursday.

In its legal action, the World Uyghur Congress argued that the NCA wrongly failed to investigate whether cotton from Xinjiang amounts to "criminal property."

Last year, a judge at London's High Court ruled there was "clear and undisputed evidence of instances of cotton being manufactured ... by the use of detained and prison labor as well as by forced labor."

But the legal challenge was dismissed on the grounds that the British authorities' approach to the law – which was that there has to be a clear link between alleged criminality and a specific product – was correct.

The Court of Appeal overturned that decision, ruling that "the question of whether to carry out an investigation ... will be remitted to the NCA for reconsideration."

Rahima Mahmut, UK Director of the World Uyghur Congress, described the ruling as "a monumental victory and a moral triumph."

"This win represents a measure of justice for those Uyghurs and other Turkic people who have been tortured and subjected to slave labor," Mahmut said in a statement.

A spokesperson for the NCA said: "We respectfully note the judgment of the Court of Appeal and are considering our next steps." (Reporting by Sam Tobin. Editing by Jane Merriman)