British police are advising major companies to tighten security as September 11 approaches in case of copycat attacks, officials said over the weekend.
Police said they were talking to "a whole range of organisations" about security, although they had no specific intelligence pointing to an anniversary attack by Osama bin Laden's al Qaeda network.
"All the big players have been approached in the previous weeks," said John Fryer of the Financial Services Authority (FSA), the regulatory body in Europe's biggest financial centre.
Britain is considered a potential target because of the government's support of Washington's 'war on terror' and because the UK is seen by some as a haven for radical Islam.
Security staff have been briefed on the types of threat they may face and will be on alert at offices across London, especially in the City and Canary Wharf, and will step up spot checks on the day, security sources said.
Organisations such as the FSA have set up secret bunkers with computers and telephones to help keep companies working and markets moving in the event of trouble. The regulator is also working with other security and government organisations.
"We had our own experience in the financial centre and City of London, experience with bombs, but in response to September 11 we must be aware of risks to every company," Fryer said, referring to an Irish Republican Army attack in 1996.
A spokesman for London's Metropolitan Police refused to discuss the specifics of any security arrangements but did refer to a "multi-agency approach across the board".
Just after September 11, British police stepped up security in "potentially vulnerable areas" of London and put more than 1,000 extra officers on the streets.
But a parliamentary report last month said Britain had failed to overhaul internal security and was poorly equipped to deal with an attack similar to that on the World Trade Centre.
Defence analyst Paul Beaver said there was still a risk from al Qaeda and copycat attackers, with Europe considered a softer target than North America.
"People have got to prepare for September 11. There is wide belief in the Middle East that al Qaeda will seek to mark the anniversary," said Beaver, who has just returned from the region. "With two weeks to go, I think it is important that companies do take measures to face that risk."
The Times newspaper said security would be bolstered at all Britain's airports, nuclear power stations and military bases. The Ministry of Defence and the Office for Civil Nuclear Security declined to comment.
Airport authorities declined to say what extra security measures they were taking. "Security is our number one priority - whatever the time of year," said a spokeswoman for the British Airports Authority, which owns seven airports, among them London's busy Heathrow.