A senior source associated with Crossrail has told the BBC the “best case scenario” is the new railway link will open in Spring 2020.
The source added, a “middle probability case” will be summer of 2020 however, the “worst case is the spring of 2021” which is more than two years after it was supposed to open.
The source further said, testing of trains and signalling was “proving more difficult than was first thought.
“It all depends on how dynamic testing goes between now and the end of this year.
“The last quarter of this year will be a critical period for the testing.”
Crossrail said the Elizabeth Line needs to be “completed as quickly as possible and brought into service for passengers.
“We are working very hard to finalise our new plan to deliver the opening at the earliest opportunity and we will be providing more details later this month.”
A spokesman for the department of transport (DfT) insisted it “consistently challenged the leadership of Crossrail Ltd” over the delivery of the project.