Crossrail faces an estimated £500m funding black hole as the Government struggles to keep the country’s most ambitious rail project on track in its final stages.

With a budget of nearly £15bn, it is already one of the country’s largest infrastructure projects. It is understood the final cost is due to rise further, however, as unexpected problems have forced unscheduled engineering.

At Paddington, for instance, a pedestrian tunnel had to be dug to allow Crossrail passengers to reach other tube lines without crossing the station concourse, a rail industry insider said. The work had not been included in the original plans.

In June last year, Paul Maynard, the then Parliamentary Under Secretary of State for Transport, told Parliament that Crossrail’s board was predicting it would be within the overall £14.8bn budget but that “cost pressures are increasing across the project”.