DAVID BLUNKETT last night issued a national terror alert, giving a warning that al-Qaeda could unleash a “dirty” nuclear bomb or poison gas attack on Britain.

The Home Secretary said that the Islamist terrorist group could use boats or trains to infiltrate cities and towns to carry out devastating bomb attacks.

He also said that terror groups could resort to the tactics long used by the IRA — assassination attempts on VIPs, car bombs in crowded centres and parcel bombs. Mr Blunkett also suggested that they might try to hijack passenger aircraft.

But within 30 minutes of the document being released, the Home Office asked journalists to return it. They then issued a blander version omitting any reference to a “dirty” bomb or poison gas.

Last night, the Home Office said the original was a week-old draft, but in Westminster it was believed that it was withdrawn because ministers did not wish to cause panic.

Intelligence sources said that the warning was not linked to a specific threat or information, but the Government is concerned to ensure vigilance is maintained at a high level in the coming weeks.There will be extra security mounted for the Service of Remembrance at the Cenotaph in Whitehall on Sunday and for the State Opening of Parliament next Wednesday.

Both events attract large crowds who come to watch ceremonies where virtually the entire Establishment, including the Queen, the Prime Minister and the Cabinet, are on public show.

Mr Blunkett gave his warning in the preface to a new assessment of the anti-terrorist measures taken by the Government since the September 11 attacks on the World Trade Centre and Pentagon in America. It was issued as he held talks with Governor Tom Ridge, the Director of Homeland Security in the US.

In the original preface to the 35-page report, the Home Secretary said: “We cannot be sure of when and where or how the terrorists will strike, but we can be sure that they will try. They may attempt to use more familiar terrorist methods, such as leaving parcel or vehicle bombs in public places, or hijacking passenger aircraft.

“However, they may try something different, perhaps as surprising as the attacks on the World Trade Centre, or the theatre siege in Moscow.

“Maybe they will try to develop a so-called dirty bomb, or some kind of poison gas; maybe they will try to use boats or trains rather than planes. The bottom line is that we simply cannot be sure.”

The blander version read: “The kind of risk that the UK is used to because of our experience with terrorism related to Northern Ireland — car bombs and parcel bombs in public places, assassinations for example — must be taken seriously, and the public must remain vigilant.

“But as we have seen with the World Trade Centre and Pentagon attacks, the theatre siege of Moscow, the attack on a French ship off Yemen, the scale of the attacks in Bali, today’s breed of terrorist is looking for ever more dramatic and devastating effects. If al-Qaeda could mount an attack upon key economic targets, or upon our transport infrastructure, they would. If they could inflict damage upon the health of our population, they would.”

The full extent of Government concern over the terrorist threat facing Britain is revealed by the scale of preparations under way in Whitehall.

The document discloses co-ordinated planning with suppliers of essential servies and the creation of a national stockpile of drugs and medical equipment to deal with anthrax, smallpox or radioactive exposure.

Ministers are creating a Health Protection Agency that would take action during a terrorist attack. Emergency services have been overhauled and hospitals will have 300 mobile decontamination units and 8,000 protective suits for use in the aftermath of a chemical or biological incident.

MI5 and the police have set up the National Counter Terrorism and Security Office to assess new terrorist tactics and develop defences. A joint unit, including MI5, Special Branch and the anti-terrorist branch at Scotland Yard, has been created to conduct operations against terrorism.

Mr Blunkett’s officials said his warning was not an EU-wide alarm, but it follows a warning in Germany, where anti-terrorist chiefs said that a militant Islamic group, al-Tawhid, are believed to be planning a new wave of terrorist attacks later this month.

Senior figures said that Germany, and its neighbours in Europe, should be on full alert for “the next 20 days” as the group is described as being “very active”. Al-Tawhid are believed to be responsible for the attack on a synagogue in Djerba, Tunisia, in April which killed 21 people.

Earlier this week German security chiefs named the London-based cleric, Abu Qatada, as the leader of al-Tawhid.

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