A £350m bailout of the delayed Crossrail project has been announced by the government.
London’s £15bn route, to be known as the Elizabeth line, had been due to open in December, but will now be launched in autumn 2019.
Transport for London said it would miss out on £20m in revenue, but the total cost of the delay could be far higher.
Rail Minister Jo Johnson, said the loan "will ensure that full momentum is maintained behind Crossrail".
The scheme, Europe's biggest infrastructure project, is currently running almost £600m over budget, and the loan has not come without criticism.
Chair of the London Assembly Transport Committee Caroline Pidgeon called the loan "only a sticking plaster to keep the project going".
"Londoners need to know what work is left to open the line, how much this will cost and ultimately who will pick up the tab," she said.
When open, the project will help ease London's chronic congestion by connecting major landmarks such as Heathrow Airport and the Canary Wharf business district.
Trains will run from Reading and Heathrow in the west through 13 miles of new tunnels to Shenfield and Abbey Wood in the east, when fully operational.
Crossrail says the new line will connect Paddington to Canary Wharf in 17 minutes and described the 10-year project as "hugely complex".
The firm said "further time" was required for testing, and that contractors needed to complete work in the central tunnels and to develop "railways systems software".
The "interim" loan will be made available to the Mayor of London following an independent review of Crossrail's governance and a separate review on Crossrail's finances by accountancy firm KPMG.
Mayor of London Sadiq Khan said: "This agreement is the result of positive discussions between City Hall, Transport for London, the Treasury and the Department for Transport and will allow Crossrail Ltd to continue their construction work and the testing process."
London's Transport Commissioner, Mike Brown, said the money would "enable Crossrail Ltd to continue its construction work and vital testing at pace to open the Elizabeth line to passengers as quickly as possible".
"The funding will go towards Crossrail Ltd completing the final fit out of the tunnels, work on stations and the extensive safety and reliability testing needed for the new systems," he said.
An estimated 200m passengers will use the new undergound line annually, increasing central London rail capacity by 10% - the largest increase since World War Two.
Leader of London Assembly Conservatives Gareth Bacon blamed Mr Khan for the delay.
"Crossrail is a subsidiary of TfL and it is a sad state of affairs when the government has to bail out the mayor who was apparently asleep on the job," he added.
However, Rail Minister Mr Johnson added: "This project is already delivering benefits for the whole of the UK through its cross-country supply chain and its UK built train fleet.
"A further update will be provided once the discussions on the financing arrangements have concluded."