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The letter, written by MI5 director general, Jonathan Evans, warns that many companies are under threat from "Chinese state organisations", and that some have already been the victim of coordinated attacks.

The article claims that Rolls Royce and Royal Dutch Shell have both been subjected to "sustained spying assaults" which aimed to steal sensitive commercial information.

MI5 will neither confirm nor

As early as 2001, the then foreign secretary, Robin Cook, warned MPs that computer espionage could be a bigger threat to the UK than terrorism. In September this year it was claimed that attacks on government computers were perpetrated by the Chinese People's Liberation Army.

The US, too, has expressed concerns about security threats from China; a US congressional advisory panel recently labeled computer attacks from China "the single greatest risk" to US technology.