"If you hit the financial center of the United Kingdom, it's a high-profile thing to do," Hart says. "It is a question of when, not if, the City is struck."

London's financial quarter houses hundreds of banks, insurance companies, law firms and other institutions--including the London Stock Exchange and the Bank of England. Aldgate subway station, one of the targets of the July 7 suicide bombings that killed 56 people including the four attackers, lies on the eastern edge of the City of London, a dense network of narrow streets dotted with skyscrapers.

Hart says police had "no specific intelligence to direct toward an attack on the City," as the district is known. But he adds: "We are always vulnerable as a financial center, as we have been for the last three decades."

The district has its own police force--distinct from the Metropolitan Police that operates in the rest of the capital--and officers increased security there in the 1990s after a string of IRA bombs. The City of London police have extended the security cordon around the Square Mile--known as the "ring of steel"--originally put in place during the IRA bombing campaign.

Hart also says "most successful terrorist operatives pre-survey their targets" and adds that this "has already occurred" but officers had disrupted "hostile surveillance." But he warns that only half the businesses in London's financial district have contingency plans in place despite a terrorist attack being inevitable.