The UK government has accused the Chinese of hacking into the computer systems of some of its leading companies, according reports published by the English press over the weekend.

According to The Times, the Director-General of intelligence agency MI5, Jonathan Evans, sent a confidential letter to 300 chief executives and security chiefs at financial institutions and legal firms last week warning them that they were under attack from Chinese state organisations.

The summary of the letter, which was posted (securely) on the Web site of the Centre for the Protection of the National Infrastructure, warned its recipients of the "electronic espionage attack".

"The contents of the letter highlight the following: the Director-General's concerns about the possible damage to UK business resulting from electronic attack sponsored by Chinese state organisations, and the fact that the attacks are designed to defeat best-practice IT security systems.

"The letter acknowledges the strong economic and commercial reasons to do business with China, but the need to ensure management of the risks involved."

According to one security expert quoted in the Times article, one of the techniques used by the Chinese groups were "custom Trojans", software designed to hack into the network of a particular firm and feed back confidential data.

The MI5 Web site already acknowledges the UK is a high priority espionage target.

"We estimate that at least 20 foreign intelligence services are operating to some degree against UK interests. Of greatest concern are the Russians and Chinese," it says.