A leaked intelligence report warned last April of a possible terrorist plot to coincide with the departure of Tony Blair, and highlighted the threat Britain still faced from Al-Qaeda.

On Friday the fears of intelligence officers were confirmed. Only extraordinary good fortune and the vigilance of emergency workers forestalled disaster in London.

The leaked document, compiled by the Joint Terrorism Analysis Centre (JTAC) and obtained by The Sunday Times, was not the only warning of the risk of a new terrorist assault on London. It emerged this weekend that nightclubs across Britain were warned in the past few days they could be the targets of car bombs.

But the public was not warned to be extra vigilant and the government is now likely to face questions over its decision not to highlight its concerns.

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BACKGROUND

• Brown plans to double 28-day detention limit

• Men in bungled raid say police attacked again

• More than Vigilance

• 'Dirty bomb' mastermind plotted wave of atrocities across Britain

• Jihadi house parties of hate

Patrick Mercer, the Tory MP and former homeland security spokesman, said: “If they had a JTAC document saying there was a high risk of an attack to mark the end of the Blair administration, why didn’t they raise the threat level and why weren’t people warned?”

The JTAC document highlighted intelligence that Al-Qaeda cells in Iran were in contact with operatives in Britain. The report stated there was no specific intelligence of an attack on British soil, but added: “Recent reporting has described [Al-Qaeda’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. We assess that this operation is most likely to be a large-scale, mass casualty attack.”

In recent weeks, a 53-page document was sent to police and then to potential targets warning of possible car bombs. Security experts say the document was not a response to a specific threat.

Earlier this year Sir Ian Blair, the Metropolitan police commissioner, said: “In terms of the greatest threat we face, vehicle-born weaponry is the greatest danger.”

Last month it emerged police were stopping and searching tankers coming into London to check for bombs. The checks were said to be precautionary.

However, Beverli Rhodes, head of security for Capita Symonds, the company that manages the capital’s “ring of steel”, said this weekend some jihadi websites suggested an attack to mark the start of Brown’s premiership.

And just hours before the car bombs were left in London, a message appeared on the Sunni Al-Hesbah chat forum. It read: “Today I say: rejoice, by Allah, London shall be bombed.”

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