Capgemini was forced to evacuate its building on Wandsworth Road and invoke its business continuity plans after a helicopter crashed 200 yards from the offices.
Around 600 sales, marketing and engineers work from the premises in Vauxhall but only security and facilities management staff had already clocked in before the copter crashed at 8am.
A spokesman at Capgemini told The Channel, that its building was roughly 200 yards from the crash but an assessment showed there was no damage.
"The immediate impact was that we were asked [at 9am] to evacuate by the police, they cordoned off the building in a half mile radius of the crash," he said.
Vauxhall Stations have been reopened but police have not yet indicated when Cap staffers will be allowed to return to site.
Two people were confirmed to have died in the accident and 12 others were hurt. Police have said it was "miraculous" that the incident was not worse.
The spokesman at Cap said its data centre was behind its offices but "all systems" were given the all the clear and support, for example from carriers including BT were unaffected too.
"It's something we'll monitor over the next 24 hours to make sure our business continuity plans are working effectively. They are at the moment," he said. ®