Image copyright Metropolitan Transport Authority Image caption The plans for a rail link from Heathrow airport to East London will be put back to 2022
The government has announced plans to delay the building of part of the Elizabeth Line by two years to 2022.
It said extensive engineering work needed to be done before construction could start.
Transport secretary Chris Grayling said the disruption would be “minor” as services would run most of the time.
The government says the new service will connect Heathrow Airport to East London.
The £20bn project will run from Heathrow Airport to a new east London station, as well as linking to Liverpool Street in central London.
As well as delays to the Elizabeth Line being pushed back, the government has also postponed the £1.6bn north-south corridor.
That will allow a route between Heathrow and central London to be opened in 2023 instead of 2026.
Mr Grayling said the public consultation for the Elizabeth Line, which would cost £8.5bn, had shown concerns about the changes.
“This is a huge transport project, the biggest ever in our country,” he said.
Image copyright Metropolitan Transport Authority Image caption The Elizabeth Line, due to open in December 2019, will run from south London to Stansted
“It involves major delays to single-track operation, which we are working through to minimise any disruption for customers.
“Transport for London [TfL] has worked hard with Network Rail to ensure that Elizabeth Line services will still be able to run almost the entire time they are delayed.”
The government said TfL and Network Rail had made a joint proposal for the start of work next spring, as opposed to the autumn or spring of 2022.
It added the framework for the work on the north-south corridor will be published in the autumn.
Approval of the plans was due by March 2018.
A final decision on the Elizabeth Line will be made by the end of March 2019.
The announcement comes less than a month after TfL celebrated the passing of its Great Eastern Main Line Phase 1 Technical and Engineering Report – a document of key performance indicators that will underpin work on building the new rail links between London and the Midlands and the east of England.