The government has announced it has stopped developing Crossrail 2.
The £41bn rail line was scheduled to open in the 2030s and run under London from the north-east to the south-west.
On Thursday the Treasury confirmed as part of its Spending Review it had agreed with Transport for London (TfL) not to fund the project
However, the London mayor's office said it planned to safeguard the route in case work could begin in the future.
The Treasury said ending the project would "free up investment" to raise the performance of public transport networks in other cities "towards London's gold standard".
"Levelling up the rest of the UK does not mean levelling London down," a departmental spokesperson added.
At the start of November, TfL agreed to end consultancy work on Crossrail 2 "as soon as possible", as a condition of the government's second £1.8bn bailout.
TfL maintains the project is "still relevant and aligned to the Department for Transport's decarbonisation plan".
The Treasury said it was "continuing to address capacity issues in the capital, by financing the completion of Crossrail".
Crossrail - to be known as the Elizabeth Line - was initially due to open in December 2018 but has faced numerous delays.
The line is due to connect Berkshire to Essex via central London.
Bosses say the project will be "shut down" without an additional £1.1bn of funding.