Feb. 25 (Bloomberg) -- China’s government ordered the closure of about 10,000 unregistered Web sites yesterday as part of an effort to “clean up” Internet content, Piper Jaffray & Co. said in a report.

Most of the affected sites will be allowed to reopen in the next two or three months after getting registered, analysts Gene Munster and Vivian Li wrote in the report dated yesterday, citing Chinese media sources they didn’t identify.

Baidu Inc., operator of China’s most-used Internet search engine, may have its first-quarter revenue reduced by as much as 2 percent because of the closures as shut sites stop advertising, the Piper Jaffray report said. Most of the closed sites belong to small businesses, it said.

Wang Lijian, a Beijing-based spokesman for the Ministry of Industry and Information Technology, didn’t immediately answer calls to his office and mobile phone this morning seeking comment.

Companies that offer Web site hosting services were told to close unregistered sites by 6 p.m. local time yesterday, according to the Piper Jaffray report. The circular sent to so- called Internet service providers ordering the closures also mandated that all new Web sites must get government approval before opening, the report said.

China censors online content judged to be pornographic, related to gambling or critical of the government by blocking access to Web sites such as those of Playboy Enterprises Inc. and the International Campaign for Tibet. All Web sites in China are required to be licensed by the state.

To contact the reporter on this story: Lee Spears in Beijing at lspears2@bloomberg.net.

Last Updated: February 24, 2009 23:02 EST