AL-QAEDA leaders in Iraq are planning the first “large-scale” terrorist attacks on Britain and other western targets with the help of supporters in Iran, according to a leaked intelligence report.

Spy chiefs warn that one operative had said he was planning an attack on “a par with Hiroshima and Nagasaki” in an attempt to “shake the Roman throne”, a reference to the West.

Another plot could be timed to coincide with Tony Blair stepping down as prime minister, an event described by Al-Qaeda planners as a “change in the head of the company”.

The report, produced earlier this month and seen by The Sunday Times, appears to provide evidence that Al-Qaeda is active in Iran and has ambitions far beyond the improvised attacks it has been waging against British and American soldiers in Iraq.

There is no evidence of a formal relationship between Al-Qaeda, a Sunni group, and the Shi’ite regime of President Mah-moud Ahmadinejad, but experts suggest that Iran’s leaders may be turning a blind eye to the terrorist organisation’s activities.

The intelligence report also makes it clear that senior Al-Qaeda figures in the region have been in recent contact with operatives in Britain.

It follows revelations last year that up to 150 Britons had travelled to Iraq to fight as part of Al-Qaeda’s “foreign legion”. A number are thought to have returned to the UK, after receiving terrorist training, to form sleeper cells.

The report was compiled by the Joint Terrorism Analysis Centre (JTAC) - based at MI5’s London headquarters - and provides a quarterly review of the international terror threat to Britain. It draws a distinction between Osama Bin Laden and Al-Qaeda’s core leadership, who are thought to be hiding on the Afghan-Pakistan border, and affiliated organisations elsewhere.

The document states: “While networks linked to AQ [Al-Qaeda] Core pose the greatest threat to the UK, the intelligence during this quarter has highlighted the potential threat from other areas, particularly AQI [Al-Qaeda in Iraq].”

The report continues: “Recent reporting has described AQI’s Kurdish network in Iran planning what we believe may be a large-scale attack against a western target.

“A member of this network is reportedly involved in an operation which he believes requires AQ Core authorisation. He claims the operation will be on ‘a par with Hiroshima and Naga-saki’ and will ‘shake the Roman throne’. We assess that this operation is most likely to be a large-scale, mass casualty attack against the West.”

The report says there is “no indication” this attack would specifically target Britain, “although we are aware that AQI . . . networks are active in the UK”.

Analysts believe the reference to Hiroshima and Naga-saki, where more than 200,000 people died in nuclear attacks on Japan at the end of the second world war, is unlikely to be a literal boast.

“It could be just a reference to a huge explosion,” said a counter-terrorist source. “They [Al-Qaeda] have got to do something soon that is radical otherwise they start losing credibility.”

Despite aspiring to a nuclear capability, Al-Qaeda is not thought to have acquired weapons grade material. However, several plots involving “dirty bombs” - conventional explosive devices surrounded by radioactive material - have been foiled.

Last year Al-Qaeda’s leader in Iraq called on nuclear scientists to apply their knowledge of biological and radiological weapons to “the field of jihad”.

Details of a separate plot to attack Britain, “ideally” before Blair steps down this summer, were contained in a letter written by Abdul al-Hadi al-Iraqi, an Iraqi Kurd and senior Al-Qaeda commander.

According to the JTAC document, Hadi “stressed the need to take care to ensure that the attack was successful and on a large scale”. The plan was to be relayed to an Iran-based Al-Qaeda facilitator.

The Home Office declined to comment.

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