The golden rule in the counter-espionage business is that when your “enemies” get more sophisticated you have to keep one step ahead of them, particularly when the threat is an electronic one.

The Chinese and Russians have now become so adept at hacking into computer systems that MI5 has had to issue a general warning to all government departments and defence companies to take extra precautions to protect their most sensitive information.

The technology section at MI5 and the powerful signals intelligence apparatus at GCHQ, the Government’s communications headquarters in Cheltenham, are on constant alert for cyber-warfare attacks against official computer systems. So advanced are the techniques now used, however, that individual companies, government departments and national utility organisations have to be advised regularly how to counter the growing threat.

Jonathan Evans, who took over as Director-General of MI5 in April, told his staff that while counter-terrorism would remain his top priority, it was important not to forget the undiminished threat from espionage, particularly from China and Russia. He made a reference to Chinese and Russian espionage attacks on Britain in his first public speech last month.

One of the most serious forms of attack that the Chinese People’s Liberation Army appears to be favouring is the use of “Trojanised e-mails”, which are delivered either in e-mail attachments or through links to a website. The e-mails use a fake sender address and information relevant to the recipient’s job or interests to entice him into opening the documents.

Once they are installed on a computer, these Trojans can obtain passwords, scan networks, extract information and launch further attacks. Antivirus software and firewalls provide some protection but Trojans can be modified to evade antivirus detection.

Although it is difficult to pinpoint the precise origin of the attacks, technicians in MI5 and GCHQ have often succeeded in tracing them to the Far East. The attacks normally focus on individuals who have jobs working with commercially or economically sensitive data.

Officials in Whitehall, as well as employees in defence companies and banks, are advised not to open e-mail attachments from any source unless they have been through antivirus scanning, and the e-mail is consistent with previous communications with the sender.

Times Recommends

• Donor organ allowed child’s own heart to heal
• Michael Jackson: Not the final chapter
• Brüno toned down for 15 rating