The NAO’s intervention follows a request from the chair of the Commons public accounts committee, Labour MP Meg Hillier, to investigate what caused costs to soar and “when it became clear that the project would be delayed”.
The NAO said: “Our investigation will examine the causes of the cost increases and schedule delays, the terms of the additional funding, and the governance and oversight of the programme.”
Mayor of London Sadiq Khan has come under fire over Crossrail (Getty Images)
Crossrail is a subsidiary of Transport for London, which Mr Khan chairs, and is jointly funded by it and the Department for Transport.
Total costs for the 73-mile east-west London link have soared to £15.8 billion. Mr Khan claims he was kept in the dark by Crossrail chiefs and has branded its internal governance as “not fit for purpose”.