Greatest threat to Britain is a flu pandemic that could kill 750,000, warns Government report

The greatest threat facing Britain is a flu pandemic that could kill 750,000 people, a Government report will warn today.

A national 'risk register' has identified an outbreak as the emergency that would have the greatest impact  -  though a terror attack is considered more likely.

The expert assessment of the dangers facing Britain, previously held confidentially within Government, is to be published by the Cabinet Office, and will be updated annually.

It also assesses other potential emergencies such as extreme flooding, cyber-attacks, storms, animal diseases and climate change events.

Officials say the move is designed to enable communities to 'prepare better' for potential disasters. Gordon Brown says the register will give the public information about risks from 'natural disasters, accidents and malicious threats over the next five years so that those who wish to can prepare for the consequences'.

Government advisers are understood to have estimated that a flu pandemic would cause between 50,000 and 750,000 deaths.

Mass graves, inflatable mortuaries, 24-hour cremations and 'express' funerals could all be used.

In a normal winter, the influenza virus kills about 12,000 people, most of them elderly.

But history shows that the flu virus can mutate into a new strain that is resistant to existing drugs.

Most famously, the Spanish flu pandemic of 1918 killed 228,000 people in the UK and an estimated 20 to 40million worldwide.

Experts say a flu pandemic is now overdue, either from the mutation of the normal human flu virus, or of bird flu.

Another major threat identified in today's report is coastal flooding on a scale that last took place along 1,000 miles of the east coast in 1953, killing 300 people.

On terror, officials say Britain is facing 30 known plots. Intelligence services are monitoring 200 terrorist networks and around 2,000 individuals are suspected of being involved.

There is a 'serious and sustained threat' from violent extremists claiming to act in the name as Islam, greater in scale and ambition than any faced in the past.

Many of the networks and individuals identified share an ambition to cause 'mass casualties without warning'. They are willing to use suicide attacks, and have 'aspirations' to use chemical, biological and radiological weapons.

In the future, Government advisers believe climate change has the potential to become the greatest challenge to global security. Extreme weather events will generate humanitarian crises and migration, they say.

The impact of global warming is already being felt, with higher temperatures and changing weather patterns. Rising sea levels and disappearing ice will alter international borders, increasing the risk of territorial disputes.

An increase in floods, droughts and storms will cause humanitarian emergencies on a new scale. Rising temperatures will also increase pressures on water supplies.

The Government is already carrying out a detailed region-by-region analysis of the impact of climate change across the UK.

It will include an examination of water and food security and supply issues. But officials warn that coastal or tidal flooding could lead to hundreds of thousands having to be evacuated and sheltered.