As a Mumbai-style attack on the UK is foiled, a leading counter-terror analyst tell Channel 4 News a random strike by a "couple of individuals" could cause "deep impact".
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Intelligence officials claim to have stopped a terror plot against targets across Britain.
The planned attacks have drawn comparison to the deadly raids in the Indian capital Mumbai two years ago, when heavily armed gunmen launched attacks on a number of targets in the city, including two luxury hotels and the railway station.
Officials said agencies across western Europe had been tracking a "succession" of terror plots by teams of militants based in Pakistan, to launch simultaneous strikes against the the UK, France and Germany.
Officials said agencies across western Europe had been tracking a 'succession' of terror plots.
US investigators are also trying to determine whether the plot extended across the Atlantic. One source told the AP news agency there was an "Islamist connection", but would not confirm whether the plot was inspired by al-Qaida.
Other sources told Reuters that initial reports of the plot, said to be in its early stages, emerged around the time of the anniversary of the 11 September attacks.
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'Individuals with handguns can cause deep impact', explains Dr Tobias Feakin from Rusi:
"What is very probable is that al-Qaida affilliated groups are considering changes of tactics because of the relative success we've had in the UK in terms of responding to the threat that we've faced... predominantly networked groups of individuals planning spectacular attacks involving explosive devices.
"It is very probable that al-Qaida operatives are changing their style of attack and this [Mumbai] certainly would represent an interesting avenue for them to explore because of the success that they've seen in Mumbai and the knid of havoc that caused.
"It becomes very seductive, if you like, as a new method of attack. It fits that modus operandi of al-Qaida of spectacular attacks that cause huge impact on the nation that they're attacking.
"In terms of the level of equipment [terrorists] need it's not an incredibly high-tech array of pieces that you need.
"Access to firearms in the UK is relatively difficult but not impossible. To acquire the kind of command and control equipment you need is not so difficult - you could use pieces of an iPhone for instance.
"What they do require is very sophisticated attack planning. What we saw in Mumbai was a lot of attack planning that took place on the ground in advance of the terrorists arriving - so that they understood the layout of the buildings, the streets and the forces they would be facing.
"Also there was a commander, if you like, in control of feeding information intto those guys, so there needs to be an external point of control as well. It's difficult responidng to these knids of attacks because of the random nature.
"In the UK we have armed response vehicles who are patrolling in London, armed police in the centre of London and special operations military on standby should something like this take place.
"It doesn't take very long to cause a great deal of impact, if you like so even if it was just a couple of individuals with handguns you can still cause a lot of deep impact in a very short space of time before it's dealt with."
According to a German counter-terrorism official, a German citizen of Afghan descent was the source of much of the information about the plot. TV network CNN said the official had named the man as Ahmed Sidiqi, who worked for a cleaning company at Hamburg International Airport. He attended a mosque in the city which had been known as a meeting place for the men behind the 911 attacks, including one of the hijackers, Mohammad Atta.
According to the official, Sidiqi and several other Germans travelled to the Afghan-Pakistan border in 2009, where he joined an extremist group linked to al-Qaida called the Islamic Movement of Uzbekistan.
He was detained in Kabul in July, and apparently "started talking a lot" - revealing details about the plot - once he was transferred to US custody.
However the Home Office did confirm that the UK's terror threat level has not been raised from its current status of "severe".
France and Germany are both on a heightened state of alert.
Yesterday French police evacuated the Eiffel tower in Paris - for the second time in as many weeks, after a threat was phoned in - although bomb experts didn't find anything suspicious.
And in the United States, a Federal official said the "volume seems to be turned up" on threat information from Western Europe - with particular concern about an "active shooter scenario" intended to create as many casualties and cause widespread chaos as quickly as possible - like the Mumbai attacks.
The US military has sharply increased its drone strikes against terrorist targets in Pakistan over the last few weeks, mostly in the dangerous border area of North Waziristan. One official told CNN they were acting on precise intelligence, adding "we would be remiss not to try to take action to thwart what might be underway in Europe".