Joint venture contractors Ferrovial Agroman Laing O’Rouke had originally planned to start the main drive this summer.

But the start of the two 650 tonne TBM drives has now been delayed to early next year when the Battersea crossover box is due to be finished to launch tunnelling.

Transport for London project engineers would not be drawn on the reasons for the delayed start, but they have stressed that the FLO joint venture would make up the time and meet the original 2020 deadline.

A TfL spokesperson said: “In a project of this scale and complexity, modifications sometimes need to be made to the work schedule.  However, the Northern Line Extension remains on target for completion in 2020.”

Both TBMs have already been manufactured in central France and are due to be dismantled and shipped to London this year.

They will then be reassembled on the Battersea site before starting their six-month 2.5km journey to Kennington.

The extension will create a spur from Kennington with two new stations at Nine Elms and Battersea.

The project will be financed by the Greater London Authority borrowing against income from incremental business rates, Battersea Power Station redevelopment contributions and the Community Infrastructure Levy.