Crossrail bosses have halted all tunnelling involving sprayed concrete following the death of a construction worker.
The 43-year-old man died after a piece of concrete fell from the ceiling of a tunnel in Holborn.
He was using a machine to spray liquid concrete onto the walls and roof of a newly-excavated tunnel beneath Fisher St.
The man, who has not been named, is thought to have suffered a fatal head injury.
A Crossrail spokesman said the accident happened at approximately 5am.
Crossrail is Europe's biggest construction project employing 10,000 workers.
Gail Cafrtmail, Assistant General Secretary of the construction workers' union Unite has demanded a meeting with Crossrail managers.
A Crossrail contractor was severely burned in 2012 when workers accidentally cut through a high voltage electricity cable on another site in Holborn.
Tom Fitzpatrick of Construction News said Crossrail was proud of its "Target Zero" safety record:
Today's tragedy is being investigated by the police and Health & Safety executive.