The deputy mayor, Wang Lijun, was reassigned last week from overseeing public security - where he was closely associated with a high-profile crackdown on organised crime - to overseeing education, science and environmental protection.

The local government's information office said it understood that Mr Wang, who had suffered from overwork and immense mental stress for a long time, was seriously indisposed physically.

It added that he was undergoing a vacation-style treatment. However, many Chinese microblog users scoffed at the statement and speculated that he had been purged. Some said Mr Wang had sought asylum at the US consulate in Chengdu.

A mainland scholar who studies party politics said the affair would be a big blow to Bo Xilai, who is widely seen as pushing for a spot in the party's central leadership, which will be decided later this year.