Disaster recovery and business continuity specialist, Global Continuity, has moved the core systems of six clients from the heart of the city.
Five more companies have put Global Continuity on standby in the aftermath of President Bush's speech on 20 September, when he told world leaders: "Either you are with us or you are with the terrorists."
Reflecting the tension, Metropolitan Police Commissioner, Sir John Stevens said: "Who are the biggest allies of America? Which is the next big target? It's got to be here."
Richard Pursey, chief executive of Global Continuity, said the company had moved more than 600 PCs and servers, which would support up to 6,000 positions.
"In situations like this we remove 10% of the business, which is the absolute minimum necessary for the business to continue. Extra purchasing of equipment can then be phased in.
"In one week we have provided infrastructure for up to 6,000 key positions. Businesses are not yet in crisis mode but they are expecting to be."
Pursey said that one client is moving "at least 30% of its staff out of London. They are logistically spreading the risk. Many believe themselves to be targets in the event of further terrorist attacks. After seeing nearly the whole team of Cantor Fitzgerald wiped out, businesses are making sure that all eggs are not in one basket."
Pursey said cost was not an issue for companies trying to ensure business continuity. "It appears to be a blank cheque," he told CW360. "They will spend what it needs to make it happen."
Stephen Bean, marketing manager for rival business continuity services provider Guardian IT, said he had not had similar requests after the US attacks.
Twenty five Guardian IT customers went on "high alert" on the day of the bombing and one major financial institution prepared to move some 2,000 employees from Canary Wharf to its recovery location centre, before calling off the alert.
"We prepared to move the staff but nothing actually happened," said Bean. A further 12 Guardian IT customers have reduced their level of alert since the US tragedy, the company told CW360.
Simon Langdon, a senior consultant for business continuity firm, Insight Consulting, said that moving staff out of central locations, "sounded a little bit like a knee-jerk reaction. There's always the danger that businesses tend to react to the last disaster. No two are ever the same. Risk analysis, good leadership and a steady hand are necessary in times of crisis," he said.