The discs will replace the standard cutting teeth which are used to drive through London clay along most of the route.

The TBM – known as Ada – has stopped 1.5 metres from the diaphragm wall of the station ticket hall to enable the switch.

Ada will then take between four and six weeks to pass through the concrete wall.

Crossrail’s other TBM – Phyllis – has already driven through to the south of the ticket hall through sprayed concrete lined tunnels.

A spokesperson for Crossrail said: “Construction of the western tunnels between Royal Oak and Farringdon is progressing well and will complete this winter as planned.

“Our lead TBM, Phyllis, has tunnelled past Bond Street and is now heading for Tottenham Court Road station.

“Our second TBM, Ada, has undertaken a planned stop, 1.5 metres from the diaphragm wall of the Bond Street western ticket hall to enable the cutterhead teeth to be replaced with discs that are designed to cut through harder material such as concrete.

“A section of the diaphragm wall is also being mined to allow for the safe passage of the cutterhead’s central cone section.”

BFK is Crossrail’s western tunnels contractor.