By Clare Baldwin

HONG KONG (Reuters) - Three Hong Kong activists have asked the High Court to review an election commission ruling that could bar anyone who advocates independence from China from standing in an upcoming poll, the first legal challenge to a decision that was supported by Beijing's representative in the city.

For the first time, candidates for September's Legislative Council election will be required to explicitly pledge that Hong Kong is an "inalienable" part of China, the Electoral Affairs Commission (EAC) said on Friday. Previously, candidates only needed to pledge to uphold Hong Kong laws.

The activists - League of Social Democrats members Avery Ng Man-yuen and Chan Tak-cheung, and Hong Kong Indigenous member Edward Leung Tin-kei - have all refused to sign the new confirmation form. They filed two separate applications for judicial review dated Sunday, according to the filings, which were seen by Reuters.

Both review requests argued that the new form was illegal and that the officers tasked with determining candidates' eligibility with regard to the new form did not have the relevant authority.

Ng and Chan also said that they objected to what they saw as "unlawful political screening of candidates in an election".

Other democratic lawmakers, including Democratic Party chairwoman Emily Lau, have also refused to sign.

The EAC declined to comment on the judicial review requests. EAC chairman Justice Barnabas Fung Wah said last week that the new confirmation form had a sound legal basis.

Beijing's chief representative in Hong Kong, Zhang Xiaoming, said separately last week that allowing independence activists to run for office - or even use the election as a platform for their ideas - would be a breach of the "one country, two systems" framework that governs Hong Kong.

Hong Kong is a special administrative region of China. It is governed by separate laws from the mainland under a "one country, two systems" framework agreed with Beijing when the British handed back the former colony in 1997.

(Reporting by Clare Baldwin; Editing by Alex Richardson)