Britain's new National Security Strategy has named terrorism and cyber-attacks on vital computer networks as the biggest threats to the UK in the immediate future.

The new National Security Strategy announced by David Cameron identifies four "tier one" risks which, it says, must be the Government's highest priority.

The other major threats are a large scale accident or natural hazard such as pandemic flu and an international military crisis which could draw in the UK and its allies.

The written statement to MP's, has been drawn up by the Prime Minister's new National Security Council as part of an assessment of Britain's defence needs which he ordered in May.

In a joint foreword to the strategy, David Cameron and Deputy Prime Minister Nick Clegg said that 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."

The launch comes just days after the head of the Government's GCHQ eavesdropping centre, Iain Lobban, warned of the very real danger of cyber-terrorism directed at the UK's critical computer infrastructure.

He said that there were 20,000 malicious emails on Government networks every month, and significant disruption had been caused to official systems by electronic "worms".

The publication of the strategy comes ahead of the release tomorrow of the Strategic Defence and Security Review, which will set out the future shape of the armed forces.