Previously secret information ranked a terror attack on transport as the most likely risk faced by the country, raising the spectre of another July 7.

The second most likely form of threat is an electronic attack on IT and communications systems, and the third most likely is a terror strike on a crowded place.

The complex analysis in the Cabinet Office's National Risk Register looked at a wide range of threats to the country, including natural disasters and industrial accidents.

Different threats are ranked on a specially-devised graph according to "relative likelihood" and "relative impact".

The document said: "Of the different malicious attacks outlined in this document, conventional attacks on transport systems are judged to be some of the more likely to occur. This assessment is supported by the many examples of this type of attack perpetrated by different groups across the globe."

Railways, including underground train systems in UK cities, are "more vulnerable to attack" than airlines because of different security levels, it said.

As announced by Prime Minister Gordon Brown in March, the document said a flu pandemic was the biggest threat facing the country. Such an incident would be the most damaging, claiming up to 750,000 lives.

But it is only the fifth most likely type of risk, according to the new information, and is ranked as slightly less likely than a severe weather attack. Coastal flooding was given as the second most damaging type of threat.

The document also provided advice to families and organisations on what they can do to protect themselves.



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