One of Europe’s most ambitious engineering projects, London's new east-west railway, Crossrail, has been delayed again until at least 2021 and could cost an extra £650 million (€750 million), planners admitted on Friday.
The £18 billion line, which will link Heathrow Airport to the Canary Wharf financial district via central London, has been repeatedly delayed by problems with safety testing and the completion of signalling systems.
It was supposed to have opened in December 2018 but deadlines have slipped several times.
The 73-mile (118-kilometer) line, which will also serve outer suburbs such as Reading in the west and Shenfield and Abbey Wood in the east, will be known as the Elizabeth Line when launched, after Queen Elizabeth.
Track, tunnels and new trains were completed in 2018, but stations and safety systems remain problematic.
In Friday's statement, Crossrail CEO Mark Wild said services would start as "soon as is practically possible in 2021."
The line, which is Britain's biggest infrastructure project in decades, is funded by the government and Transport for London.