London's Crossrail project might now not open until the autumn of 2021, according to Transport for London commissioner Mike Brown.
The project was originally supposed to be finished in December 2018 but has suffered several setbacks.
Last year Crossrail Ltd admitted that the line would probably not open until the spring of 2021.
Parts of the route, named the Elizabeth Line, are running already, but the central section is yet to open.
"The assumption we've made is, I suppose, at the pessimistic end, but it's at the pragmatic end and you would expect us to take that approach," Mr Brown told the London Assembly.
The cost of the project could reach £18.25bn, more than £2bn more than the original budget.
The cost was originally set at £15.9bn for the scheme, which will connect major landmarks such as Heathrow Airport and the Canary Wharf business district.
A TfL spokesperson said: "The Crossrail team continues to make progress completing the railway and is moving forward with the complex testing of the signalling and train systems so that the railway can be handed over safely and reliably for passenger service. As Crossrail Ltd previously announced, the Elizabeth Line will open as soon as practically possible in 2021.
"As part of our annual business planning process, we have made some prudent assumptions including that the central section of the railway could open in autumn 2021, but continue to support Crossrail Ltd in delivering the railway as soon as possible. Crossrail Ltd continue to refine their delivery schedule and will provide an update in the coming weeks."