LONDON  British business executives dealing with China were given a formal warning more than a year ago by Britain’s security service, MI5, that Chinese intelligence agencies were engaged in a wide-ranging effort to hack into British companies’ computers and to blackmail British businesspeople over sexual relationships and other improprieties, according to people familiar with the MI5 document.

The warning, in a 14-page document titled “The Threat from Chinese Espionage,” was prepared in 2008 by MI5’s Center for the Protection of National Infrastructure, and distributed in what security officials described as a “restricted” form to hundreds of British banks and other financial institutions and businesses. The document followed public warnings from senior MI5 officials that China posed “one of the most significant espionage threats” to Britain.

Details of the document were confirmed Sunday by two people familiar with its contents, who both spoke on an anonymous basis because of the sensitivity of the subject. The document’s existence was first reported in the British newspaper The Sunday Times.

Last month, Google announced that it was considering ending its operations in China after a “sophisticated and targeted” cyberattack that it said aimed primarily to gain access to the e-mail accounts of Chinese human rights activists. Google said it was no longer willing to cooperate with China in what amounted to censorship of its search engine, which Google had operated in a way that prevented millions of Chinese from reaching Web sites deemed hostile by Beijing.