In a briefing in the City of London 12 days ago, a senior detective from SO15, the Metropolitan police counter-terrorism command, said: “Mumbai is coming to London.”
The detective said companies should anticipate a shooting and hostage-taking raid “involving a small number of gunmen with handguns and improvised explosive devices”.
The warning — the bluntest issued by police — has underlined an assessment that a terrorist cell may be preparing an attack on London early next year.
During a “commando-style” raid by 10 gunmen on hotels and cafes in Mumbai in November 2008, 174 people were killed and more than 300 injured over three days.
Officials now report an increase in “intelligence chatter” — communications captured by electronic eavesdropping agencies. One senior security adviser said the police warnings had intensified and become much more specific in the past fortnight.
“Before, there has been speculation. Now we are getting what appears to be a definite plot to carry out a firearms attack on London,” he said.
Earlier this year, police, military and intelligence services held an exercise in Kent to see whether they could defeat a commando raid in London by terrorists.
“The exercise brought out to those taking part that the capability doesn’t exist to deal with that situation should it arise,” said a military source.
Security sources said concerns had been raised by “chatter” on a prominent jihadist website two weeks ago.
One contributor suggested fighters could use automatic weapons to strike places such as nightclubs, sporting venues and Jewish centres.
In an online discussion hosted on December 2, another contributor invited suggestions for carrying out “guerrilla warfare” and proposed “a group of mujaheddin raid police stations and fire at them”.
Another said: “Make sure that all those at the location are of age, that there are no children and so on. Insist on the locations and times where no Muslims or children are to be expected.
“If machine guns are available, and explosive and expertise for [explosives] are not available, this is a good way ... The [Mumbai] operation is the ideal scenario for operations you are talking about.”
A third contributor said targets should be “chosen in a studied manner”.
He added: “In general, targeting economic joints and intelligence centres if possible has priority over police stations.”
The Met is understood to be struggling to draw up effective plans to deal with the challenge of mass shootings followed by a prolonged siege with terrorists prepared to kill their hostages and themselves.
In Mumbai, many victims were killed in the first half hour of the attack. The Met is concerned that it will be much longer before the SAS, which has traditionally dealt with terrorist sieges in London, would arrive from its base at Regent’s Park barracks.
Patrick Mercer, chairman of the Commons counter-terrorism sub-committee, said the threat was “very real”.