INTELLIGENCE intercepts of al-Qaeda suspects have uncovered a level of terrorist plotting on the same scale as in the weeks leading up to the September 11 attacks, The Times can disclose.

The intense level of al-Qaeda “chatter” picked up by American and British signals interceptions led to David Blunkett’s sombre warning of a terrorist attack.

The intelligence gleaned in recent weeks pointed directly to a terrorist threat against Western interests, and the United Kingdom is believed to be on the list of targets.

In the more alarmist version of Mr Blunkett’s assessment of the threat that was put out by the Home Office in error — and then rapidly retrieved — the Home Secretary gave a warning of the possibility of Britain being attacked by terrorists armed with a “dirty” radiological bomb or poison gas.

Yesterday, Mr Blunkett said that a “clerical error” had been made and the wrong assessment had been published. The version subsequently published made no reference to “dirty” bombs. Mr Blunkett said that he did not want to risk creating “unjustified panic and disruption”.

The evidence which has emerged, however, from the latest intelligence makes it clear that the threat to Britain is high and Government officials saw the need to let the public know. Tony Blair was involved in the decision to publish the terrorism document.

Both drafts of the statement, the one warning of a potential dirty bomb or poison gas attack and the other using more bland language, had been formally approved by the security and intelligence services, indicating that the first was as legitimate as the second.

However, the “dirty” bomb scenario has become more of an obsession with the United States. This was made clear by Governor Tom Ridge, President Bush’s Homeland Security Advisor, when he met the heads of all the British agencies in London this week.

The main concern in British intelligence circles is the potential threat from suicide bombers. The Bali bomb came as a severe shock because it demonstrated that al-Qaeda had switched to attacking a soft Western target. There are fears that a similar attack could be attempted in Britain.

The reason for publishing the Home Office document was to encourage the public to be vigilant. “There have been many cases where members of the public have alerted the police when spotting a suspect package and we needed to inject the same sense of vigilance for the latest threats we face,” one Whitehall official said.

Anxiety over growing complacency of a terrorist threat has driven Mr Blair to make it the subject of his annual speech at the Lord Mayor’s banquet on Monday.

The Prime Minister will say the need for vigilance is as great as immediately after the September 11 attacks and urge the public to be on its guard.

Although the threat to the City and other prominent locations in the country is “qualitatively different” than that posed by the IRA in the 1970s and 1980s, the public response should be the same, he will say.

His advisers say he is trying to strike the right balance between alerting the public to the danger and triggering the sort of panic that followed the September 11 attacks.

One of the most alarming warnings of a terrorist outrage in Europe came from Germany. Hans-Josef Beth, head of the International Terrorism Department of the Security Service (BND), has identified Abu Mussab al-Zarqawi, a one-legged terrorist with experience in chemical warfare, as the likely mastermind of a future assault.

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