The government could take over the reins of Crossrail from its fellow joint sponsor Transport for London (TfL) if a lid is not kept on the costs of the £15.4bn project.

The Sunday Times first reported that in Crossrail's sponsor agreement, signed in 2008, there is a clause that entitles the Department for Transport (DfT) to take over the project if costs exceed certain "intervention points".

On 24 July the rail minister Jo Johnson announced that project, also known as the Elizabeth Line, needed an extra £600m in funding, taking its total cost from £14.8bn to £15.4bn.

Over a month later it was announced that the line would not open in December, as scheduled, but in the autumn next year.

Read more: Sadiq Khan accused of misleading public over nine-month Crossrail delay

The Elizabeth Line is a huge source of income for cash-strapped TfL, which is straddling a £1bn operational deficit and is counting on the project to bring in £926m in annual income by 2022. If the project were to be handed over to the government it would be a further blow to TfL, which will already miss out on an initial £20m from the delay to the opening, as well as £10m from advertising.

A spokesperson for TfL said: “Everyone involved in this project is fully focused on ensuring the Elizabeth Line is completed as quickly as possible and brought into service for passengers. We are working closely with Crossrail Ltd and government on delivering this. Crossrail Ltd is working to establish what the additional impact on funding from the revised delivery schedule will be and discussions continue with government on how to deliver any financing that may be required.”

A DfT spokesperson said: “Crossrail Ltd and TfL are working to identify the level of additional funding that will be required to complete the project and how this will be provided.”

Read more: TfL to lose out on £10m in advertising from Elizabeth Line delay