The new £15bn Crossrail route in London will open almost a year later than planned after missing its December opening date.
Those in charge of the project admitted on Friday an extension is needed to finish "final infrastructure and extensive testing" to ensure a "safe and reliable railway for customers".
The major scheme, which has been under construction since 2009, will be known as the Elizabeth line once it starts carrying passengers.
Services were due to begin running in December this year, but its central section – between Paddington and Abbey Wood – will now not be opened until autumn 2019.
The original timetable has been abandoned because more time is required to complete work in the central tunnels and develop software.
The full east-west line will be opened "as soon after the the central tunnels open as possible", the project said.
Simon Wright, chief executive of Crossrail Limited, said: "The Elizabeth line is one of the most complex and challenging infrastructure projects ever undertaken in the UK and is now in its final stages.
"We have made huge progress with the delivery of this incredible project but we need further time to complete the testing of the new railway.
"We are working around the clock with our supply chain and Transport for London to complete and commission the Elizabeth line."
Once finally completed, the railway will take passengers from Abbey Wood, in London's south east, to Heathrow Airport to the west of the capital.
Services are also planned to run from Shenfield in Essex to Reading in Berkshire.
Last month, rail minister Jo Johnson announced Crossrail's budget was being increased from £14.8bn to £15.4bn due to "cost pressures".
He hailed "another productive year" for the scheme and claimed Crossrail was now "93% complete" prior to its testing and commissioning stage.
Labour peer Andrew Adonis, a member of the Department for Transport when Crossrail's construction began, used the news to continue his attacks on current Transport Secretary Chris Grayling.
In a withering series of tweets about the "debacle", Lord Adonis wrote: "Govt just announced, on day 39 of Parliament on holiday, that Crossrail is being delayed by a year and they have big problems with signalling & cost over-runs
"This is huge story & smuggling it out on last Friday of August a classic ruse. More Grayling catastrophe."
Mr Grayling has recently been under pressure over train chaos in the north of England.
As well as Crossrail, the government is also planning to deliver other major infrastructure projects such as the HS2 railway linking London, Birmingham, Leeds and Manchester, as well as a third runway at Heathrow.
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There are also proposals for a north-south Crossrail 2.
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