The modernisation of four tube lines is to be delayed by up to five years following the embarrassing failure of a £350m project to install new signals.
The high-tech upgrade of the Circle, District, Hammersmith & City and Metropolitan lines was due to finish in 2018.
But Transport for London today confirmed a revised target date of 2023 for the new system.
TfL claims the improvements will mean 'faster and more frequent' services with trains every two minutes in central London.
The original £354m contract with Bombardier was scrapped in 2013 after tube bosses concluded the system might not work.
The project has now been handed to a new supplier, Thales, with a revised price tag of £760m.
TfL admits it is unable to meet the original 2018 target for work on all four 'sub-surface' lines to be finished.
We have a very clear delivery plan and timetable for the work and, as we have done with the modernisation of the Northern line, we will keep London moving and growing as we do it. In parallel, we will continue to deliver a better, more reliable service every day which builds on the work over recent years to reduce delays to their lowest ever level.
With rush hour tubes bursting at the seams, and commuters struggling to get into work every day on overcrowded services, upgrading the tube is undoubtedly the single most important transport project in London.