James Hart said some big international corporations with offices in London’s so-called “square mile” have failed to take the threat seriously.
“Every successful terrorist group pre-surveys its target,” the commissioner of the City of London police told the Financial Times newspaper.
“There’s no doubt we’ve been subjected to that surveillance,” he said.
Potential targets included iconic sites, business and prominent buildings –“anywhere where the maximum damage can be inflicted on the financial systems of the City of London,” he said.
Despite the July 7 attacks on three Underground subway trains and a bus that killed 56 people, only half of businesses in the City have contingency plans in place, he said.
The July 7 attacks have been linked by the British government to Osama bin Laden’s al-Qaeda network.
Meanwhile a man is due to be charged in relation to the failed July 21 bomb attacks on London.
Abdul Sharif, 28, from south London, is due in court on Thursday, charged under Britain’s Terrorism Act, London’s Metropolitan Police said.
He’s charged with failing to disclose information about Hussain Osman, who is suspected of trying to blow up an Underground train near Shepherd’s Bush station on July 7.
And in a related development the United Arab Emirates on Wednesday denied a press report that radical Muslim cleric Omar Bakri Mohammed, who faces the prospect of treason charges in Britain, was in the country.
A Saudi-owned Arabic newspaper had quoted Bakri saying he had arrived in Sharjah on Tuesday after spending three days in Beirut.
The Syrian-born imam, who also holds a Lebanese passport, told BBC radio he intended to return to Britain to defend himself against the charges.
Bakri is well known by British authorities for his hardline Islamist views.
He sparked outrage in Britain last week when he said he would not inform police if he knew Muslims were planning a bomb attack on a train in Britain. He also supported Muslims who attack British troops in Afghanistan and Iraq.
Prime Minister Tony Blair has vowed to crackdown on hardline Islamists in the wake of the July 7 suicide bombings.