The rail scheme, one of the largest construction projects in Europe, is drifting substantially over budget

An estimated £500m bailout of the Crossrail train line is expected to be announced within weeks, to prevent work on the colossal infrastructure project from grinding to a halt, The Sunday Times can reveal.

Faced with problems over the power supply and signals, the £14.8bn London rail scheme — one of the largest construction projects in Europe — is drifting substantially over budget.

Contractors are racing against the clock to complete the line in time for its scheduled December opening. The budget overspend will raise more questions about Britain’s ability to deliver mega- projects on time and to budget.

Work on a string of huge schemes, from the £20bn Hinkley Point nuclear power station in Somerset to the £56bn HS2 rail line and Heathrow’s £14bn