Transport for London will borrow £350 million the Department for Transport.
The group are facing a shortage in funding following delays in the £15 billion Crossrail project.
Rail Minister Jo Johnson said: “Today, as an interim measure, we are announcing that £350 million of short term repayable financing will be made available to the Mayor for the year 2018/19.”
“This will ensure that full momentum is maintained behind Crossrail.”
“When open, Crossrail will be transformative and carry up to 200 million passengers a year, delivering £42 billion of investment into the UK economy,” he added.
Interest rates and terms of the loan are still being finalised, which will decide to what cost Londoners will face from borrowing from the government.
The launch of the Crossrail was initially planned for this year but has been pushed back to autumn 2019 after delays.
Paul Scully, who is the Conservative party’s vice-chairman for London, said: “This is emblematic of Sadiq Khan’s complete mismanagement of TfL’s finances, which he is in charge of.”
“It beggars belief that on the day the government has had to bail him out, he is off grandstanding in Brussels,” he said, referring to Khan and Michel Barnier’s meeting on Friday.
Sadiq Khan commented that the loan would “allow Crossrail Ltd to continue their construction work and the testing process”.
“An independent review into Crossrail Ltd’s commercial practices, cost projections and governance is already well underway, and that will help us understand the exact implications of the project over running.”
“Some very challenging work remains to be completed,” he said.
The overall cost of the project has risen from £15.4 billion to approximately £15.75 billion.
London’s Transport Commissioner, Mike Brown, said: “The confirmation of an interim financing package between the Government and the Mayor of London will 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.”