The Government tonight pledged to transform Britain's defences to counter cyber attacks as it warned of the "devastating real-world effect" of a successful assault on the UK's communications infrastructure.
David Cameron laid the ground for major spending cuts to the armed forces - due to be announced tomorrow - with the publication of a National Security Strategy (NSS) focusing heavily on the threat of terrorism and cyber attacks.
With warships, fighter jets, tanks, and thousands of troops all facing the axe in Strategic Defence and Security Review (SDSR), the NSS called for a "transformative programme" to combat the cyber threat from "states, criminals and terrorists".
The threat of cyber attack was listed in the NSS as one of four "tier one" priority risks to the UK alongside terrorism, a major accident or natural hazard such as pandemic flu, and an international military crisis.
While al-Qa'ida and its affiliates still represented the "principal" terrorist threat - including the possibility of an attack with chemical, biological, radiological or nuclear weapons - the NSS also highlighted the threat from dissident Irish republicans.
It is the first time that a national security strategy has drawn up a priority list of threats, based on an analysis of the likelihood and impact of all existing and potential threats over a five and 20 year time horizon.
The "tier two" threats are attack using weapons of mass destruction, overseas conflict which terrorists can exploit to threaten the UK, a significant increase in organised crime, and disruption to satellite communications.
A large-scale conventional military attack on the UK is rated only as a "tier three" priority alongside disruption to oil and gas supplies and a large-scale radioactive release from a civil nuclear site.
Other "tier three" risks include a significant increase in the numbers of terrorists, organised criminals or illegal immigrants trying to enter the UK, an attack on another Nato or EU member or a UK overseas territory, and the disruption of essential supplies such as food or minerals.
While NSS said there was still a need for the UK to be able to "project power" abroad to protects its interests at home, critics will see it as cover for cuts, with up to 8% set to be axed from the defence budget.
Although the UK needed to be capable of responding in the event of a military crisis, the NSS said the focus of its diplomatic, intelligence and defence capabilities should be on preventing such crises materialising in the first place.
In a joint foreword, Mr Cameron and Deputy Prime Minister Nick Clegg said there needed to be a "radical transformation" in the way that Britain thought about and organised its national security.
"We are entering an age of uncertainty. This strategy is about gearing Britain up for this new age of uncertainty - weighing up the threats we face and preparing to deal with them," they said.
"As a Government, we have inherited a defence and security structure that is woefully unsuitable for the world we live in today. We are determined to learn from those mistakes and make the changes needed."
Officials acknowledged that the focus on cyber warfare was relatively new. "If we were doing this five years ago, we would not have put cyber security up as high as it is," said one senior official.
It follows a public warning by Iain Lobban, the Director of the GCHQ electronic spy agency, that government computers were receiving 20,000 "malicious" emails a month - 1,000 of which were deliberately targeting them.
The NSS said that the risks emanating from the growing dependence on the internet were "huge" with government, businesses and individuals all coming under "sustained cyber attack".
It highlighted the "stuxnet" computer worm - which has appeared on the computer system of the Iranian nuclear programme - as an example of the damage which such attacks can cause.
"Attacks in cyberspace can have a potentially devastating real-world effect. Government military, industrial and economic targets, including critical services, could feasibly be disrupted by a capable adversary," it said.