The rail scheme faces huge cost overruns, but it's not the only big project behind schedule and over budget.Read more
BBC personal finance correspondent
Signing off all the safety aspects of the Crossrail project before handing over to Transport to London is taking longer than expected.
It is also taking more time to test the signalling software on the line.
The stations are nearly complete, with the exception of Bond Street and Whitechapel.
The hope is that most will be finished early next year, with Whitechapel completed later but Bond Street at risk of not being ready for when the line opens.
The cost of the project is likely to rise by between £400 and £650m, taking it to a possible £18.45bn.
Crossrail - running from Heathrow Airport to the Canary Wharf - will not open until 2021. It was supposed to open in December 2018.
Mark Wild, Crossrail chief executive, said: "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," said Mark Wild, Crossrail chief executive.
"The Elizabeth line must be completed to the highest safety and quality standards."
"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."
Earth dug up for the rail line's tunnels and stations has provided a habitat for birds and seals.
London's Crossrail project will probably go even further over budget, according to a report by MPs.
Commuters have been "let down" by a programme that is well behind schedule, the Public Accounts Committee said.
MPs said they were "sceptical" about the Department for Transport's "ability to oversee major rail projects".
In response, the Department for Transport said it had acted "swiftly and effectively" when problems at Crossrail became clear.