London’s £15bn Crossrail project - now named the Elizabeth line - is to open in autumn 2019, nine months after its scheduled launch.
The revised schedule is 'needed to complete the final infrastructure and extensive testing required to ensure the Elizabeth line opens as a safe and reliable railway', Crossrail Ltd said in a statement.
The company, which is wholly-owned by Transport for London, added that more time was 'needed by contractors to complete fit-out activity in the central tunnels and the development of railway systems software'.
The scheme is Europe's biggest infrastructure project and is already running around £600m over budget.
When fully operational, trains will run from Reading and Heathrow in the west through 13 miles of new tunnels to Shenfield and Abbey Wood in the east.
It will operate as three separate services - as previously planned.
Simon Wright, Crossrail Chief Executive 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.'