Scotland Yard says it has uncovered evidence that al-Qaida plotted to bring down the internet in the UK.
The plot, which could have caused widespread disruption to businesses, banks and consumers, was discovered in a number of raids by the police.
Scotland Yard told the Sunday Times that it discovered computer files suggesting terrorists were to target a high-security internet hub in the Docklands area of London.
The site, Telehouse Europe, houses several servers and is described as Europe's biggest "web hotel" containing information that makes up the world wide web.
Security officials confirmed that the country was put on a heightened state of alert at about the same time as the bomb plot was uncovered last year.
They add that the plot against the internet hub reflects the changing threat from al-Qaida and associated terrorist groups.
A senior Whitehall official said: " The Telehouse facility was the subject of intense reconnaissance. The evidence suggests that it was one of a range of options considered by the suspects."
It was one of several "infrastructure" facilities targeted by terrorists, with telecommunications and the internet high on the agenda.
MI5 says that without such infrastructure, the UK "could suffer serious consequences, including severe economic damage, grave social disruption, or even large-scale loss of life".
Dame Eliza Manningham-Buller has now set up a special MI5 unit called the Centre for the Protection of National Infrastructure in order to deal with threats to infrastructure.
Robert Harris, technical services director at the hub, added: "Security and business continuity are critically important. Our industry remains as alert as possible to any threat, terrorist or otherwise, and we are in regular communication with the appropriate authorities."
"The climate in 2006 required a heightened state of alert. In 2007 we remain in this heightened state of awareness to any such security threat and are in regular dialogue with the authorities."
The news comes as the Internet Services Providers' Association (ISPA) has been called on to give evidence to the House of Lords' science and technology committee.
Growth in broadband use and the growth of internet banking has prompted an inquiry into online security, which the ISPA will now aid.
Jessica Hendrie-Liano, chair of the ISPA Council, said: "It is important that the nature of the internet is understood and the success of the industry's hard work to date is acknowledged."