The cost of modernising signalling on four Tube lines has more than doubled.
Transport for London (TfL) has awarded a £760 million contract to upgrade the District, Circle, Metropolitan and Hammersmith & City lines to global transportation systems provider Thales.
It originally gave Bombardier a target price of £354m to deliver the work in 2011, but the contract was terminated.
TfL said the prices were "not comparable" because of inflation and the increased development work.
The work will also be completed four years later than planned in 2023.
The District, Circle, Metropolitan and Hammersmith & City lines will receive new tracks, longer platforms, a new signalling system and rebuilt depots under the modernisation plans.
Nick Brown, managing director of London Underground, said: "Having successfully modernised three of the most heavily used lines on our network, we are ready to begin work to bring the next four lines in to the modern era.
"This will transform the journeys of millions of our customers, significantly increasing service reliability and frequency."
The company argues the previous contract price with Bombardier did not reflect the large amount of work required to adapt their technology for use on the Underground.
TfL said the 2010/11 prices would now be £388 million in 2015/16 prices.
In 2014 the London Assembly Transport Committee criticised TfL for paying £80m to Bombardier after it failed to deliver the modernisation contract.
Work is expected to begin later this year and the main benefits will be delivered by 2022, when the frequency of trains running during peak periods will increase to 32 trains an hour in central London.
TfL said the plans for a new signalling system include the oldest part of the network, which was built in 1863 and "belong in a museum".
Labour's London Assembly transport spokeswoman Val Shawcross said: "The Tube upgrade should have topped the mayor's list of priorities, but his abject failure to stay focused now means that paying passengers will be left to contend with yet another five years of over-stretched services."
The overall budget for the four line modernisation programme is £5.41 billion, TfL said.