The four bidders in the race for the deal were Balfour Beatty/Bam, Bechtel/Strabag, Costain/Murphy and Laing O’Rourke/Ferrovial.

The Enquirer understands that it is now a straight race between Balfour Beatty/Bam and Laing O’Rourke/Ferrovial.

David Hughes, Director of Director Of Major Programme Sponsorship for London Underground, said: “Following the issue of an OJEU notice on 15 March last year and the subsequent pre-qualification process, we invited tenders from four bidders for the design and build contract for the Northern Line Extension.

“We received high quality tenders and, following the tender evaluation process, we have invited the two highest scoring bidders to participate in the next phase of the procurement process.

“This process will involve assessing their ability to deliver the best value for money and to meet delivery expectations.”

TfL said it couldn’t comment on the identity of bidders during an ongoing procurement process.

But sources close to the project have confirmed the revised shortlist to the Enquirer.

A contract is expected to be awarded later this year for the extension from Kennington to Battersea.