Thales was pre-selected for the contract in April 2014 after it presented a solution that meets the intricate operational requirements of the lines following LU's termination of its £354m 2011 contract with Bombardier. The new system will enhance capacity by a third on the lines, which make up 40% of the tube network.

Thales is now entering the final stages of negotiation with LU to deliver the new system, and LU's expectation is that the cost of work will be in line with, or below, the cost per-km of the Northern Line signalling upgrade and significantly less than the Jubilee and Victoria line modernisations delivered under through public-private partnerships.

LU says it expects that the cost of the contract will be higher than the Bombardier deal in which it was supplying a communications-based train control (CBTC) solution and had completed preparatory work. However, LU says with work not progressing and with no guarantee that it would work, it decided to act to avoid the failure of the project.

LU is investing £5.54bn to modernise the lines, which includes delivery of 191 air-conditioned S-Stock trains from Bombardier, which are already in use on the Metropolitan, Circle, District, and Hammersmith & City lines as well as new track, lengthened platforms and rebuilt depots.

This is an increase on the original £4.26bn budget to account for "a more realistic price for the signalling contract, a longer overall programme, and additional infrastructure and costs identified as necessary following the termination of the Bombardier contract."

Once completed the signalling upgrade will increase frequency of trains on the Circle Line to every four minutes instead of 10 minutes at present, and there will be a service every two minutes across much of the SSL network when additional District, Hammersmith & City, and Metropolitan line services are added.

LU says that once work on the four lines is complete in 2022 it will move on to purchasing new trains and control systems for the Piccadilly, Central, Bakerloo, and Waterloo & City lines.

"Having modernised three of the most heavily-used lines we now need to bring the next four lines in to the modern era," says Mr Mike Brown, LU managing director. "To do so is absolutely vital to us continuing to effectively support London's new economic growth, housing and jobs as London's population grows from 8.6 million today to 10 million in 2030."