The long-awaited Elizabeth Line which was due to open in just four months time has been delayed by almost a year.
Crossrail announced that "further time is required to ensure a safe and reliable railway" meaning Londoners won't be able to experience the new 13 mile line from west to east.
This is despite promises earlier this year that the Elizabeth Line which will add a grey line to the Tube map will open in Decembe r - just in time for Christmas.
According to Transport for London (TfL) a "revised schedule is needed to complete the final infrastructure" and "extensive testing is required to ensure the Elizabeth line opens as a safe and reliable railway."
As previously planned, the 10-year project will run from Reading and Heathrow in the west through 13 miles of new tunnels to Shenfield and Abbey Wood in the east.
The new line will operate as three separate services - from Paddington to Abbey Wood, Paddington to Heathrow and Liverpool Street to Shenfield.
A new Elizabeth Line Station will also be created at Paddington, meaning it will only take commuters 17 minutes to travel from the new station to Canary Wharf.
"Crossrail Limited has been working hard to maintain the programme and sufficient testing time is required to introduce the next phase of the railway - the central section between Paddington and Abbey Wood in a way that can be guaranteed to be safe and reliable," a TfL spokesman 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.
"The focus remains on opening the full Elizabeth line, from Reading and Heathrow in the west to Shenfield and Abbey Wood in the east, as soon after the central tunnels open as possible."
According to TfL, the new line will add 10% to central London’s rail capacity and boost the economy by an estimated £42 billion.
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."
Mark Wild, London Underground and Elizabeth line Managing Director added: "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."