BEIJING—China’s Communist Party placed under investigation a senior official once seen as a possible successor to President Xi Jinping, people familiar with the matter said, in a change that allowed the Chinese leader to promote an ally.

A hastily arranged meeting of party officials on Saturday in the inland city of Chongqing announced that Sun Zhengcai, the city’s top official, was being replaced and was being investigated, a person familiar with the matter said. A second person corroborated the investigation.

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