Transport for London (TfL) expects to spend a further £432M of public money to complete the Northern Line Extension in south London.

In December New Civil Engineer revealed that the project would be delayed by nine months to a year and is now due to open in September 2021.TfL subsequently confirmed the delay in board papers released later that month.

In new papers released this month by the Greater London Authority (GLA) – which provides TfL with funding to build the extension – TfL is forecast to spend £213M in the 2019/20 financial year, £127M in 2020/21 and a further £92M in 2021/22 to complete the extension.

At the end of the 2018/19 financial year, it is predicted that around £690M of public money will have been spent on the project meaning the final cost to the taxpayer will come in at around £1.1bn, 10% higher than the original £1bn cost estimate.

TfL has not released the overall projected cost for building the line. It said the figures are commercially sensitive, as part of the cost will be picked up by the private developer of Battersea Power Station.

Funding from the GLA to TfL is budgeted to stop at the end of the 2020/21 financial year with TfL planning to complete the project with £92M of its own money.

Tunnelling for the 3.2km long extension from Kennington to Battersea was originally due to start in summer 2016 but was pushed back six months as modifications were made to the work schedule. It was then due to start in March 2017 but slipped to April 2017 as readying the TBMs took longer than expected.

The project has also been dogged by controversy about costs. In 2016, London Underground said it was in discussions with the developer of Battersea Power Station over funding for changes to the design of the Malaysian-backed redevelopment scheme and the design of the underground station box.

A Ferrovial, Agroman and Laing O’Rourke joint venture selected by London Underground in 2014 is carrying out the work to design and build the new line which will have stations at Battersea and Nine Elms.

The Northern Line shut down to complete the modernisation of Bank Tube station had been planned to be carried out in 2020. However, additional exploration and archiving of Roman ruins, and the need to divert utilities on around the work site meant the construction timetable had to be amended.

At Bank, around 1.2km of new tunnels are being built to reduce congestion at the station. The modernisation project is due to finish in 2022.

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