Project bosses confirmed that the line will now “open as soon as practically possible in 2021.”

Transport for London said a more exact timetable will be unveiled “early in 2020″.

Crossrail was originally due to open in December 2018

Latest cost projections are for at least a £400m hike on current budgets while “further modelling scenarios consider even higher levels of risk of £650m.”

Mark Wild, Chief Executive, Crossrail Ltd said: “The Crossrail project has made good progress over recent months as the new plan to complete the Elizabeth line is implemented by the supply chain.

“A key focus during 2019 has been finalising the stations, tunnels, portals and shafts.

“By the end of the year, Custom House, Farringdon and Tottenham Court Road stations will be complete and the project is on track to finish fit-out of the tunnels in January.

“The central section will be substantially complete by the end of the first quarter in 2020, except for Bond Street and Whitechapel stations where work will continue.

“The two critical paths for the project remain software development for the signalling and train systems, and the complex assurance and handover process for the railway; both involve safety certification for the Elizabeth line.

“These must be done to the highest quality standards to ensure reliability of the railway from day one of passenger service.

“Crossrail Ltd will need further time to complete software development for the signalling and train systems and the safety approvals process for the railway.

“The 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.

“Our latest assessment is that the opening of the central section will not occur in 2020, which was the first part of our previously declared opening window.

“The Elizabeth line will open as soon as practically possible in 2021. We will provide Londoners with further certainty about when the Elizabeth line will open early in 2020.

“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 £400 million to £650 million more than the revised funding agreed by the Mayor, Government and Transport for London in December 2018.

“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.”