Crossrail has been delayed by almost a year, the infrastructure firm behind the project has admitted today.
The Elizabeth line, which had been expected to launch in December, will now open through central London in autumn 2019.
It is understood the company only told Transport for London and the Department for Transport about the severe delay yesterday.
"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 Limited said.
"The original programme for testing has been compressed by more time being needed by contractors to complete fit-out activity in the central tunnels and the development of railway systems software. Testing has started but further time is required to complete the full range of integrated tests," it said.
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.”
A DfT spokesperson said: “We are disappointed by the delay to the opening of the Crossrail central section. However, the safety of passengers and staff and reliability of services is the overriding priority and we accept Crossrail Limited’s assessment that more time is needed to fully test the railway before passenger operations can commence.
“We remain confident that Crossrail will deliver a great improvement to passengers’ journeys once it is fully operational.”
Mark Wild, London Underground and Elizabeth line managing director, said: “We will continue to work closely with Crossrail Limited as they complete the remaining railway infrastructure work and testing needed to deliver the new railway.
"The delayed opening is disappointing, but ensuring the Elizabeth line is safe and reliable for our customers from day one is of paramount importance.”
Colin Stanbridge, chief executive of London Chamber of Commerce and Industry (LCCI) said: "It is very disappointing to hear that a revised schedule is now needed, at this stage, to complete the project’s final infrastructure.
“Londoners and the business community need the Elizabeth Line to address overcrowding and congestion within the capital.
“Of course, safety is of paramount importance, however we will hope that it may become possible for the project to be up and running before next summer”.
Jace Tyrrell, chief executive of New West End Company, said: “We are disappointed that the opening of this major and vital piece of infrastructure is being delayed. However we appreciate and fully support the need to ensure that high safety standards are met.
“But retailers throughout the UK are facing difficult trading conditions with rising costs, changing shopping habits and falling consumer spending, partly because of Brexit uncertainties.
"The economic benefits expected from the Elizabeth line were a welcome boost for West End retailers and would have had a positive impact on stores throughout Britain."
Tyrrell said the delay "increases the urgency" for government support of retailers, in particular to cut business rates.
“The postponement of the Elizabeth line opening makes it even more imperative that government acts now to support the nation’s high street businesses," he said.