Titan Rain - how Chinese hackers targeted Whitehall

· Foreign Office among departments hit 

· Military involvement suspected

Cyber Attacks are an Act of War

Chinese hackers, from the People's Liberation Army, have been attacking the computer networks of British government departments, the Guardian has learned.

The attackers have hit the network at the Foreign Office as well as those in other key departments, according to Whitehall officials.

The Ministry of Defence declined yesterday to say whether it had been hit. An incident last year that shut down part of the House of Commons computer system, initially believed to be by an individual, was discovered to be the work of an organised Chinese hacking group, officials said.

Security and defence officials are coy about what they know of specific attacks. However, they say several Whitehall departments have fallen victim to China's cyberwarriors. One expert described it as a "constant ongoing problem".

The disclosures came after reports that the Chinese military had hacked into a Pentagon military computer network in June. The Financial Times said American officials called it the most successful cyber attack on the US defence department.

Defence department officials confirmed that there had been a "detected penetration" of elements of the email system used by the network serving the office of Robert Gates, the US defence secretary. US officials were reported to have said that an investigation had discovered that the People's Liberation Army (PLA) was responsible.

The US gave the codename "Titan Rain" to the growing number of Chinese attacks, notably directed at the Pentagon but also hitting other US government departments, over the past few years.

The latest attack caused some minor administrative disruptions, but there had been no adverse impact on operations, an official said.

Angela Merkel, Germany's chancellor, is reported to have raised the issue of Chinese attacks on her government's computers during a visit to Beijing. Officials here declined to say whether the British government had raised the issue with the Chinese authorities.

Alex Neill, China expert and head of the Asia Security Programme at the Royal United Services Institute, Rusi, said cyber attacks by the Chinese had been going on for at least four years. He described the reported attack on the Pentagon as the "most flagrant and brazen to date".

He said such attacks reflected a new doctrine of the PLA described as "pressure point warfare" - the attacking of specific nodes to leave the adversary paralysed.

The incidents should be seen against the background of the forthcoming 17th Chinese Communist party congress, which could determine the next generation of leaders, and the PLA keen to flex its muscles, Mr Neill suggested.

The attacks on the Pentagon's computer system were described by Dr Sandra Bell, head of Rusi's homeland security department, as "very much a wake-up call". She added: "The Chinese see no difference between asymmetric warfare and conventional warfare".

Analysts have argued over the seriousness of the attacks, and China has officially denied responsibility. However, the latest attack was said by officials and analysts yesterday to be the most serious discovered so far.

Responsibility for advising government departments on how to protect their networks rests with MI5, GCHQ, and the Centre for the Protection of the National Infrastructure in the Cabinet Office.