Crossrail might not open to passengers until 2021, more than two years after services were originally due to commence, it has been claimed.

A senior source associated with the new railway line between Reading and Shenfield told the BBC the “best-case scenario” would be the new link opening in spring 2020.

The £17.6billlion project, also known as the Elizabeth Line, would serve stations across east London, Essex, Kent, Berkshire, and west London.

It was originally planned to open last December but this was put back until the end of this year.

However a “middle probability case” would be the summer of 2020, while a “worst case is the spring of 2021”.

The source told the BBC that 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,” they said.

“The last quarter of this year will be a critical period for the testing.”

Crossrail said London needs the Elizabeth Line 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.”

The scale of the potential delays was reported after an influential group of MPs demanded the project’s failures are explained by the Government.

Passengers had not been told the root causes for the railway being delayed and over budget, a scathing report by the Commons’ Public Accounts Committee (PAC) stated.

The cross-party committee urged the Department for Transport (DfT) to make clear what it, Transport for London and Crossrail Ltd are responsible for in relation to the project.

Committee members warned they were “not convinced” that trains would begin running in 2020 or that the “additional £2.8 billion of funding provided will be enough”.

The DfT was also accused of failing to put in place robust governance arrangements.

Crossrail’s cost is being met by the Government, the Greater London Authority - including TfL - and London businesses.

Crossrail Ltd, a wholly-owned subsidiary of TfL, was established in 2001 to build the new railway.

The project’s budget has fluctuated from £15.9bn in 2007 to £14.8bn in 2010.

But due to the cost of the delayed opening, a £2 billion Government bailout of loans and cash was announced in December.

Labour MP Meg Hillier, who chairs the PAC, said passengers have been “badly let down” and it is clear the planned opening date was “unrealistic for some time”.

A DfT spokesman insisted it “consistently challenged the leadership of Crossrail Ltd” on the delivery of the project.