The London Stock Exchange evacuated its headquarters and City workers were sent home as news sunk in of the US terrorist attacks.

Many staff had friends and colleagues in the World Trade Center towers and spent the afternoon desperately trying to contact them.

In spite of the sombre mood, City of London police said there was no immediate risk: "We have received no information and no threats."

Companies contacting the City of London Corporation were told to follow standard security procedures. They were not being advised to evacuate buildings although, as the afternoon proceeded, many chose to send employees home.

The London Stock Exchange evacuated its Exchange Tower headquarters, but contingency arrangements meant it was able to continue trading until the close.

Staff at Tower 42, the City's tallest building, were told to leave the building.

Most banks in the City did not evacuate, but told employees in non-essential jobs, or those not involved in trading, they could leave early.

The Metropolitan Police said it was telling companies to follow standard security procedures and was not recommending "anything special".

Several floors of the main tower at Canary Wharf in the east of London were evacuated following a fire alarm on the 19th floor.

A number of companies in the complex chose to let employees leave early.

Many staff remained around the Canary Wharf buildings in a state of un- certainty and shock.

"We just want to know what's happening to our colleagues in New York," a Bank of New York employee said.

City Airport said it was on highest alert following the attacks. "We will be implementing extra security measures," it said.

Companies and police were giving a consistent message on Tuesday afternoon - that security measures already in place would suffice.

Financial buildings have been a favoured target for terrorists for more than a decade because of their vulnerability and political sensitivity.

The IRA switched its attention to the City in 1992 when it bombed the Baltic Exchange, killing three.

The blast, on April 10, came just after the general election and was one of the biggest bombs ever exploded on the British mainland. Damage from the attack was estimated at £800m.

It marked a change in strategy coming just a month after two attacks in Warrington, Cheshire - one of which went off in a shopping centre killing two boys, aged three and 12.

In the wake of the Baltic Exchange bomb security was increased - but the IRA struck in the same area just a year later with the Bishopsgate bomb, which killed one person. The repair bill from that attack was £350m.

That bomb, coming so soon after the Baltic Exchange attack, led to a much wider review of security in the City. A security cordon, dramatically reducing access points to the City, and guarded by police was introduced.

It was Canary Wharf - part financial, part commercial - that was chosen by the IRA in February 1996 when it ended its ceasefire.

"The City of London was very sleepy about disaster procedures until the first IRA bomb," said Andrew Grant, head of operational risk at TCA Consulting.

He said that following the second IRA bomb, companies and the authorities had all implemented more rigorous procedures.