Earlier this year, plans were set out to complete outstanding works on the scheme within an expected delivery window between October 2020 and March 2021.

Chief executive Mark Wild has now indicated that the Elizabeth Line will not open in the first half of this window but rather “as soon as practically possible in 2021”.

He also revealed: “Our detailed cost forecasts continue to show that the project’s costs will increase due to programme risks and uncertainties.

“The latest projections indicate a range of between £400M to £650M more than the revised funding agreed by the Mayor, Government and Transport for London in December 2018.”

This agreement at the end of last year put the overall funding envelope for Crossrail at £17.6Bn.

Work on the project during 2019 has involved finalising the stations, tunnels, portals and shafts, with the central section on track to be substantially complete by the end of the first quarter of 2020, except for Bond Street and Whitechapel stations.

But the scheme needs further time to complete software development for the signalling and train systems as well as the safety approvals process for the new railway, Mark Wild said.

“A trial running phase will begin at the earliest opportunity in 2020, this will be followed by testing of the operational railway to ensure it is safe and reliable.”

He added: “We are doing everything we can to complete the Elizabeth line as quickly as we can but there are no short cuts to delivering this hugely complex railway. The Elizabeth line must be completed to the highest safety and quality standards.”

Crossrail’s latest setback comes amid speculation that an independent review into High Speed 2 will urge the Government to go ahead with the project in full, despite estimated costs rising to £88Bn.

The Oakervee review – which leaked this week – is not expected to be officially published until after the General Election.

(Photograph: Crossrail)