Nearly 300 UK government departments and businesses which are critical to the country's infrastructure have been the subject of high-tech "Trojan horse" attacks.
Many were thought to originate in the Far East, according to an alarming new report from the National Infrastructure Security Coordination Centre (NISCC).
The NISCC said it had noticed a "recent increase in sophistication" in electronic attacks on the infrastructure which includes financial, telecommunications, energy, health and transport organisations.
The attacks are launched from bogus unsolicited emails. These contain a "Trojan" attachment which seem harmless at first glance but once opened allow a remote attacker to gain full control of the user's machine.
The normally secretive NISCC, which protects the infrastructure, took the unusual step of making the highprofile announcement because of the scale of the threat. The warning appears on the NISCC website --www.niscc.gov.uk.
The Home Office confirmed the NISCC, which issues regular advice, had been working with the National Hi-Tech Crime Unit.
A spokesman said: "We wanted to take that further because there are businesses on the periphery of the critical national infrastructure that can be targeted by these attacks."
This could include banks, insurers and other financial units.
NISCC director Roger Cumming said: "This is not a few hackers sitting in their bedrooms trying to steal bank account details from individuals. This is aimed at organisations, targeted at gaining information, and is extremely well organised and well structured."
NISCC has now urged all businesses to strengthen their security systems.
Many of the difficult-tospot rogue emails have been sent from addresses in Asia.
They are targeted at people whose jobs involved dealing with the sensitive data, the NISCC said.
They are spoofed to appear as though they come from a credible source, or a trusted contact such as a news agency or government department.
The hackers use distribution lists to target large numbers of people with similar interests.
No significant information has been stolen, but the scale of these attacks appears to be increasing.
Security specialist Sophos said it had seen a threefold increase in the number of keylogging Trojans alone in the last year.
"We see more than a dozen new pieces of malware capable of stealing highly valuable and sensitive information every day," said Carole Theriault, security consultant at Sophos.
" Trojans which allow unauthorised remote access to a computer pose a serious risk to all businesses. While it's worrying that the UK's critical infrastructure could be at risk, this also serves as a reminder to all UK businesses that they must keep their systems secure to avoid confidentiality leaks.
"Every computer used for business must be protected with up-to-date anti-virus software and a firewall."
She added: "Malicious code is increasingly being written not just to cause a nuisance, but to steal money - whether targeting individual users of online banking or massive global corporations and government institutions."
Earlier this year an attempted hack on the London office of the Sumitomo Mitsui bank was blamed on keylogging Trojans and just last month a couple were arrested in London for their alleged involvement in an industrial espionage scandal targeting Israeli companies.
Sophos assisted the NISCC with its investigation by analysing the samples and providing information on how the threats work.
* To speak to Sophos to gain more information about the specific Trojans used in these attacks, contact Johnson King on 0207 357 7799.