Mark Wild told the London Assembly transport committee there is “a huge amount to do” and that fit out of the tunnel has yet to complete, but said dynamic testing will restart this week.

He added: “I don’t think there has been any lack of transparency; what happened is the enormity and complexity of Crossrail was not fully understood.”

Crossrail was scheduled to open in December but it was announced at the end of August that the date would not be met.

The project’s former chairman Sir Terry Morgan – who resigned at the end of last year – told the committee that he had said to the Mayor on 26 July that delivery of the project in 2018 was not feasible.

But the Deputy Mayor of London Heidi Alexander said she did not accept Sir Terry’s account of that meeting. “We were told (the opening date) was at high risk,” she claimed, “but he certainly hadn’t taken a decision to rule out a December opening”.

Crossrail’s new chairman Tony Meggs told the committee that the project may have suffered from a lack of “external review and assurance”. He added: “It will certainly be a priority of mine to make sure all the major things have had the right level of scrutiny.”

Following the hearing, Transport Committee chair Caroline Pidgeon said: “There are still contradicting accounts of who knew what and when.

“It continues to be a very frustrating process getting to the bottom of the confusion, but Londoners have a right to know. Not only do we need to see Crossrail back on track and open for passengers, we also need to ensure lessons are learnt for the future.”

(Photographs: Crossrail and TfL)