An outspoken Hong Kong politician who advocates independence for the financial and trade hub wrote to Donald Trump, calling on the U.S. president to get China and Hong Kong expelled from the World Trade Organization.

Andy Chan Ho-tin, a member of the Hong Kong National Party, has been criticized by Chinese and Hong Kong authorities for his stance in favor of independence for his city, which they say will never happen. His party is under threat of a ban.

Chan's letter, which does not mention Trump by name, but is addressed to "Mr. President," follows a closely watched speech and press conference last week. In that, he accused China of treating the city as a colony, blasted local officials for enabling an erosion of local rights and autonomy since Chinese rule commenced in 1997 and called on the United States to extend its trade war with China to Hong Kong.

"We are most grateful to see that the US administration under your leadership has been determined in implementing its tariff and trade policies against China, which has abused its trade relationship with the U.S. for years," Chan said in the letter, dated Saturday.

He said that Hong Kong's separate membership in the WTO effectively means that Beijing has an extra seat and vote at the organization.

"This is obviously unfair and unjustifiable," said the letter, a copy of which was provided to CNBC by the Hong Kong National Party. Chan said in it that he had sent a previous one to the president in February last year.