Fears of an al-Qaeda plot to massacre thousands in Britain appeared to heighten on Sunday night.
The threat of a Mumbai-style machine gun assault is now rated so severe that the US is warning its citizens they could be in danger if they travel to the UK.
At the same time, Britons are being told by the Foreign Office that they could face “indiscriminate” attack from Osama bin Laden-inspired fanatics if they travel to France or Germany.
US intelligence says gunmen plan to open fire on crowds at busy sites in London, Paris and Berlin - mirroring the raid on hotels and bars in Mumbai which killed 175 in 2008.
Areas said to be at risk are tourist destinations, hotels, public transport hubs and shopping centres. The City and the Tower of London are said to have been identified as possible targets.
It came amid a warning from two former SAS commanders that if terrorists did manage to strike, the country would face a “slow motion” September 11.
Lieutenant General Sir Graeme Lamb and Colonel Richard Williams said a Mumbai-type plot would “outmatch” the police and turn London into a war zone.
Police would be unable to respond if the gunmen got inside an iconic target such as Canary Wharf, they said. And they warned of innocents being “murdered one by one, floor by floor”.
According to US officials, an al-Qaeda fanatic they have under interrogation identified the Tower of London and a major mainline rail station as potential targets.
Ahmed Sidiqi, a German national who was seized in Kabul, also named the Eiffel Tower and Notre Dame Cathedral, in Paris, and three landmark targets in Berlin.
He is said to have told interrogators about the existence of cells of armed fanatics in European cities.
US intelligence chiefs say Bin Laden personally ordered the commando attacks on Britain.
In a sign of how seriously the intelligence is being treated, the Home Office, Foreign Office and US administration on Sunday delivered co-ordinated warnings to the public. The UK terror threat is at severe - meaning an atrocity is considered “highly likely”.
Analysts said it was extraordinarily rare for the US State Department to issue a Europe-wide warning of a potential terror attack.
The alert is one step below a formal travel warning advising Americans not to visit Europe.
The British government said the US advice was “consistent” with its own analysis of the threat to the public here.
In its own statement, the Foreign Office said: “Like other large European countries, the French/German authorities continue to consider that there is a high threat of terrorism. Attacks could be indiscriminate, including in places frequented by expatriates and foreign travellers.”
Home Secretary Theresa May said: “As we have consistently made clear, we face a real and serious threat from terrorism. We work closely with our international partners in countering terrorism and the US advice is consistent with our assessment.”
Whitehall officials say there are currently no plans to raise the UK terror threat rating to “critical”.
This level was last reached after the failed car bomb attacks on a London nightclub and Glasgow Airport in summer 2007.
Reports in the US suggest terrorist teams have completed their preparations and surveillance and are now ready to strike.
The US travel advice could still have a significant impact on the recession-hit tourist industry if unnerved Americans opt against travelling to Britain.
The suspects involved in the plot include British citizens of Pakistani origin. GCHQ is said to have identified a number using voice-mapping technology. Reports suggest they have intercepted suspected terrorists speaking in Midlands and Rochdale accents.
Two British brothers of Pakistani origin - one of whom was reportedly killed in drone strikes in Pakistan earlier this month - and eight Germans, were said to have been receiving terror training for the raids in camps in Pakistan. - Daily Mail