The new Crossrail service through London will miss its completion date by nine months.
Transport for London (TfL) has announced that work on the £15billion project, which was due to fully open in December this year, will overrun into autumn 2019.
Some trains are already in operation on the Elizabeth line – as the service has been named – namely between Shenfield and Liverpool Street, and between Paddington and Hayes & Harlington.
Last month TfL also revealed that costs were already running £600m over budget, and that additional finance was required to complete the route that will eventually connect Reading with Abbey Wood.
Simon Wright, Crossrail chief executive, said more time is required to ensure a safe and reliable service for passengers.
He 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.”
It is estimated that the Elizabeth line will bring an extra 1.5m people to within 45 minutes of central London, and more than 200m passengers are expected to use it every year.