The head of the firm building London's Crossrail project has admitted projected costs have risen further amid continued delays to completion.
The east-to-west Elizabeth line route, running below central London, was due to open in December 2018 but has been beset by a series of problems from construction to testing.
It was announced in April that services would begin between October 2020 and March 2021.
But in an update Mark Wild, chief executive of Crossrail Ltd, said the first trains would now only begin "as soon as practically possible in 2021" with services between Paddington and Abbey Wood.
No deadline has been set for services across the whole network - from Reading and Heathrow in the west to Abbey Wood and Shenfield in the east
Mr Wild also revealed that the cost of the railway could reach £18.25bn - a rise of £650m on the latest budget agreed between London Mayor Sadiq Khan, the government and Transport for London.
The original budget was set at £15.9bn in 2007.
Crossrail Ltd said there were four "major tasks" outstanding.
They included building software to integrate trains with three different signalling systems and the installation and testing of station, tunnel and communications systems.
Mr Wild said: "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."
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."
He issued the update in the wake of a scathing report by MPs in the summer.
The Public Accounts Committee warned then that it could be 2022 before the introduction of passenger services given the scale of the work outstanding.
Commenting on the latest delay Caroline Pidgeon, who chairs the London Assembly's transport committee, said: "The further delays to the opening of Crossrail and increasing costs are appalling news for Londoners.
"The delay creates huge issues for many businesses that have made investment decisions based on its original opening date of December 2018."