The 43-year-old man was pronounced dead at the scene after suffering head injuries ten metres below ground in the tunnel at Fisher Street, Holborn.
He was spraying concrete onto the floor of the tunnel when a piece fell from the ceiling.
Main contractor on the western tunnels section is BAM, Ferrovial, Kier (BFK).
Six fire rescue units, three fire engines and over 50 firefighters and officers were called to the scene.
A Crossrail statement said: “Sadly, at approximately 5am this morning a contractor working at our site in Fisher Street, London was fatally injured. Our first and foremost thoughts are with the family and friends affected by this. The family has been informed.
“When the incident occurred, work was taking place to build a new crossover tunnel between two train tunnels that have already been constructed by Crossrail’s boring machines.
“The worker was spraying concrete onto the excavated ground when a piece of concrete from the ceiling of the tunnel fell and hit him.
“Crossrail has suspended all work at Fisher Street and associated works across the project. A full and thorough investigation into this tragic incident is being undertaken. The Metropolitan Police and the Health and Safety Executive remain on site.
“Safety is our number one priority and Crossrail sets the most stringent safety requirements in the industry. Despite this terrible incident, Crossrail’s accident rate is below the industry average.”
It is the first fatality on the £15bn project.