The partners saw off the remaining rival bid from the Balfour Beatty/BAM joint venture to take the massive tunnelling and stations job.

Subject to approval from Transport Secretary Patrick McLoughlin, which is expected this autumn, construction is now due to start in spring 2015 and finish in 2020.

A Transport for London spokesman said: “Following negotiations with the two remaining bidders for the Northern Line Extension design and build contract, London Underground has selected Ferrovial Agroman Laing O’Rourke joint venture as the preferred bidder.

“Contract award is planned for late August. Other tenderers have been notified and should we fail to reach agreement with the preferred bidder London Underground may reopen negotiations with those who have been stood down.”

The new tunnels will link to the existing railway south of Kennington station from a section of track used by terminating trains known as the Kennington Loop.

The new extension will run 3.1km along the northbound section and 3.25km on the southbound route including crossover and stabling tunnels west of the terminus at Battersea.

The new stations will be at Nine Elms, where a new US embassy and residential high-rise schemes are being built and at Battersea next to the vast iconic power station redevelopment.

Cross-section of Battersea station terminus

Nine Elms station

Images Courtesy of Studiodare Architects