The first stage of the lengthy process of operating Crossrail services has been put back, Transport for London has admitted.
The first Bombardier class 345 train to be used on the Elizabeth Line was due to start running on existing timetables between Liverpool Street and Shenfield this month.
But tests are taking longer than expected, and it is now going to be June at the earliest before the train enters service.
TfL Elizabeth Line operations director Howard Smith said: “The train is undergoing thorough testing, assurance and approvals before it enters passenger service.
“Good progress is being made and we anticipate testing being completed within the next few weeks. We continue to work closely with Bombardier, Network Rail and MTR Crossrail.”
TfL Rail began operating services between Liverpool Street in the City and Shenfield, Essex, back in 2015.
It will take over the Paddington to Heathrow section in West London next spring.
The central London Crossrail tunnels are scheduled to be open by the end of 2018, with full services running from Reading in Berkshire to Abbey Wood in south-east London and Shenfield by the end of 2019.
Crossrail construction began in 2009, and is now more than 80 per cent complete. More than 15,000 people have worked on the £15bn project, including 600 apprentices.