The Enquirer understands that Unite union rank and file organisers are pushing for bonus payments of up to £5 an hour.

It is understood they have already won a victory over weekend working at the Woolwich site in south London and are now looking to flex their muscles across the whole project.

One site source said: “Everyone knows fit out is the crucial stage of projects like this – just look what happened on the Jubilee Line Extension.

“Unite are stepping up their campaign on Crossrail and are confident of winning better terms across the whole project.”

That will alarm other contractors keen to avoid industrial relations problems or soaring wage bills.

The source said: “The weekend working concession will make all other firms on the job very jittery.”

Crossrail tunnels and stations are being fitted out ahead of train services starting to run through central London and Docklands in December 2018.

Work includes installation of track, tunnel ventilation, high voltage power, traction power, signalling, communications and overhead line equipment.