Businesses across Britain were put on red alert today when a Government agency warned of possibly the biggest co-ordinated attack on computer systems by hackers yet seen.

One senior computer security consultant even warned that most companies' anti-virus software and firewalls would be unable to cope with the hacking assault.

The National Infrastructure Security Co-ordination Centre (NISCC) issued its warning overnight, with one official saying, "We have never seen anything like this in terms of the industrial scale of this series of attacks."

The attacks are delivered by an apparently innocent e-mail which, when opened, contains an attachment which can launch a sophisticated program - or Trojan horse - into the recipient's computer system, where it allows the hacker total access.

More than 300 government departments plus businesses in the telecommunications, banking and finance, energy and transport sectors have been targeted by the hackers, who are believed to be based in Asia.

Some attacks have also been aimed at businesses in the United States, Europe and Australia.

According to NISCC, the aim is to steal commercially and economically valuable information. It says that the hackers are becoming increasingoly sophisticated, and that they are able to construct viral programs with which anti-virus software cannot match.

Earlier this month, one British hacker, Gary McKinnon, appeared in court accused of carrying out the "biggest military computer hack of all time". Mr McKinnon, who is on bail until a further hearing next month, faces extradition to the United States if it is proved that he caused $1 million-worth of damage after hacking attacks on the Pentagon and more than 50 other US military systems in 2001 and 2002.

And last month, on the day after the General Election, a bogus story about the Prime Minister was used by hackers as a ruse to trick computer users into visiting sites hosting "malware".

Recipients of the "BBC: Tony Blair email account hacked!" messages were invited to visit a site that claimed to offer a screenshot of "Blair's hacked account". In reality, visitors were sent to a site that harbours a series of Trojan horses that aimed to steal confidential information from infected PCs, including sensitive online banking account details.

Of the latest attacks, NISCC warned business to take the threat very seriously.

"This is not a few hackers sitting in their bedroom trying to steal bank account details from individuals," Roger Cumming, the director of NISCC, said.

"The attack is clearly not targeted at stealing money. It is aimed at gathering information. It is extremely well organised and requires quite a lot of resources to execute.

"Our philosophy is that if everyone in the UK was to adopt our advice and install all the latest patches, that attack would not have any impact on UK Plc," Mr Cumming said. He confirmed that confidential warnings had already been issued to certain key sectors, and that "we have succeeded in making the UK a hard target to attack".

Paul King, Cisco System's principle security consultant, confirmed that NISCC's warning should be taken very seriously. But he also warned that the targeted nature of the attacks means that most anti-viral software would not deal with the incoming Trojans.

"The attackers have taken this to a new level," Mr King said. "They've used social engineering and a lot of research to make sure that they target their attacks. They are very speedy, and the recipients will receive an e-mail that appears to have an entirely valid and plausible context.

"That is the escalation that makes it worrying, because it makes it much more likely that people will click on the attachment which contains the Trojan.

"Anti-virus and firewalls won't always work in this scenario, because they only protect against known attackers. What is needed is something to protect against unknown attackers."

Mr King advised businesses to look into their system protection for mails without a signature, and suggested that Cisco Network users could check their Netflow, which could offer an indication of possible system compromise.

For more information from NISCC, click here

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