The opening of the central section of the £15.4bn Elizabeth line has been delayed until autumn 2019, nearly a year later than originally planned.
Crossrail revealed the line from Paddington to Abbey Wood would now not open in December this year, as the organisation needed to complete the line’s final infrastructure and finish the testing required.
The delivery body said the original programme for testing had been revised due to contractors needing more time for completing fit-out activity in the central tunnels and the development of railway systems software.
It added that testing had started but further time was required to complete the full range of integrated tests.
Crossrail chief executive Simon Wright 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.”
Last month rail minister Jo Johnson revealed that Crossrail’s funding envelope would be increased to £15.4bn, up from the £14.8bn previously set aside.
Crossrail was handed £300m to be spent on the installation of extra testing of systems on the central section of the line.
Network Rail was also given £290m to upgrade the existing rail network supporting the Elizabeth line.
Earlier this year, TfL revealed the project was under significant “cost and schedule pressures”, which had led to the rephasing and scheduling of works.
The first budget agreed for the Crossrail scheme was set at £15.9bn back in 2007.
Following a Comprehensive Spending Review in 2010, this was reduced to £14.8bn. Of this spend, Crossrail is responsible for £12.5bn of works, while Network Rail is responsible for £2.3bn.