Crossrail 2 is set to cost over £40bn, the Mayor of London has revealed in a budget document published earlier this week.
The £41.3bn estimate for the proposed north to south London line is higher than the Greater London Authority's widely cited £30bn budget, which is based on 2014 prices.
But the real cost is set to be much higher as it estimates the actual construction cost of the project.
In his final budget for 2019-2020, London mayor Sadiq Khan has put aside £10.4bn for the project between 2023 and 2028, followed by £18.4bn between 2029 and 2033 and £12.5bn from 2033 to 2038.
The estimate does come in under the £45.3bn actual cost estimate cited in a 2016 funding document.
Crossrail 2, which is scheduled to open in the 2030s, is a proposed route in the south east which would run from nine stations in Surrey to three in Hertfordshire, across London.
It would connect the South Western main line to the West Anglia main line, via the busy London stations at Victoria and King's Cross St Pancras.
Over the same 15-year period, Transport for London (TfL) will spend £18.8bn on line extensions, while £12.1bn will be spent on upgrades to TfL lines, and £28.2bn for enhancements and renewals across the network.
TfL currently plans to extend the Northern and Bakerloo lines, while upgrades include replacing trains and signalling systems across the four 'deep Tube' lines – the Piccadilly, Bakerloo, Central and Waterloo & City lines.
However, last December TfL confirmed that the pressure placed on its finances by the delay to the £17bn Elizabeth Line meant it would have to press pause on key transport schemes.
"While TfL is planning to follow the introduction of new trains on the Piccadilly Line with new signalling, and then to upgrade the rest of the deep Tube lines, such large-scale investment will not be possible without capital funding from the government," it said at the time.