LONDON (AP) -- Pandemic flu, not terrorism, is the most serious risk to the U.K. public, says Britain's first ever national threat assessment, published on Friday.

The document, part of Prime Minister Gordon Brown's overhaul of homeland security strategy, considers the likely dangers posed by threats including terrorism, climate change, extreme weather and pandemic disease.

Britain's Cabinet Office -- which drafted the document -- said on Friday that a potential flu pandemic poses the most imminent danger over the next five years.

Previous government assessments have suggested an outbreak could cause as many as 750,000 deaths in Britain and acknowledged it could take several months to develop adequate vaccines against a particular strain of the virus.

Brown ordered a list of threats faced by Britain to be drafted shortly after he replaced Tony Blair in June 2007, arguing that previously classified assessments should be made public.

Though the new register does not rank threats in order of seriousness, it does indicate that flu is considered the most pressing concern, a spokeswoman for the Cabinet Office said on Friday.

"It looks at the whole range of risks and looks at them from a national perspective," said the spokeswoman, on condition of anonymity in line with policy. "It is the first time all of this has been brought together in this way."

Brown said in March that Britain was increasing its defenses against terrorism, but also preparing for potentially more serious risks from climate change and disease.

It followed a warning from Richard Mottram, who retired in November as Brown's chief adviser on intelligence and security, that the risks of a global flu pandemic, the impact of mass global migration and threats from organized crime were receiving too little attention.

Mottram claimed that the U.K.'s security strategy is too tightly focussed on terrorism.

But Britain's domestic spy agency MI5 says it is monitoring a constantly changing network of around 2,000 potential terrorists in the U.K., who are planning around 30 potential attacks at any given time.

Jonathan Evans, head of MI5, has acknowledged that the workload means officers are neglecting key counterespionage duties -- despite an active spying threat from China and Russia against the U.K.

Under Brown's new security strategy, the agency will grow from around 3,000 staff to 4,000, and resources and technology at GCHQ, the government's secret eavesdropping center, will be upgraded.

A new advisory panel of security experts has also been created to report to Brown on threats faced by Britain.

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