HONG KONG — The Financial Times said in an editorial Sunday that the expulsion of one of its journalists from Hong Kong sends a chilling message about the steady erosion of basic rights in the semiautonomous Chinese city, as more than 15,000 people signed an online petition calling for an explanation from the government.

The newspaper said Friday that Victor Mallet, its Asia news editor, was told his work visa would not be renewed. It is believed to be the first such expulsion of a foreign journalist since Hong Kong, a former British colony, returned to Chinese control in 1997.

Mr. Mallet returned to Hong Kong after an international trip on Sunday and was allowed to enter the territory on a seven-day visitor visa, The Financial Times said. As a British citizen he would typically be allowed to stay in the territory for up to six months under a tourist visa.

“Immigration officials did not provide an explanation for the shortened visitor visa, and we continue to seek clarification from the Hong Kong authorities about the rejection of his work visa renewal,” the newspaper said in a statement.