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 Shiite regime of President Mahmoud Ahmadinejad, but experts suggest that Iran’s leaders may be turning a blind eye to the terrorist organisation’s activities.

Click here to view the full Times of London report.

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

The report was compiled by the Joint Terrorism Analysis Center (JTAC) — based at MI5’s London headquarters. It draws a distinction between Usama 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.

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”.

The Home Office declined to comment.