Risk Waters has not been able to contact any of its staff
Earlier British officials said at least 15 Britons were thought to be missing.
CF and ESpeed
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London-based Cantor-Fitzgerald, its recently spun-off subsidiary eSpeed, and Risk Waters Conferences all had offices or were working in the building at the time of the attack.
Another company ICAP, formerly Garban Intercapital, the world's largest inter-dealer broker, said it believes almost all of its employees escaped.
It is also believed there were many UK citizens working amongst the 435 companies that were tenants in the World Trade Center.
In New York, UK officials said about 15 UK citizens may be among those missing.
But in London, UK Prime Minister Tony Blair said "significant numbers" of British citizens would be among the dead.
Still missing
Cantor-Fitzgerald and eSpeed had operations on the 101st and 103rd to 105th floors, and employed 1,000 people in one of the buildings.
"The London offices of Cantor Fitzgerald International and eSpeed International are doing everything possible to assess the situation at their world headquarters at One World Trade Center in New York City," the company said on Wednesday.
"Currently we are unable to determine how many employees were in the building at the time of the attack or confirm if they were safely evacuated."
At the time of the attack Waters Conferences, a UK-based international market research and conferences company, was hosting its Financial Technology Congress on the 106th floor of the World Trade Center in New York.
"We are not able to give exact details about the numbers of people who were at the event at this time but in the region of 180 people were due to attend. Of this number, we are unable to confirm if they managed to evacuate safely," the company said in a statement on Wednesday.
The company has established that some of those registered to attended had not turned up when the planes hit.
ICAP, formerly Garban-Intercapital, the world's largest inter-dealer broker, has said its entire New York offices located on three floors of the World Trade Center were destroyed and that only a small number of staff are still unaccounted for.
Mike Sheard, director of corporate affairs, told Reuters the group had 675 people working at the building.