Up to £30 million per week is being spent on the delayed Crossrail project, it has emerged.
Auditor KPMG calculated the figure after carrying out an independent review into London’s new east-west railway.
The biggest impact on the final cost will be how long contractors working on stations continue to be paid, the report found.
Crossrail, which will be known as the Elizabeth Line when it opens, was due to launch in December but this did not happen due to several major issues.
No new opening date has been announced.
Crossrail Ltd’s chief executive, Mark Wild, revealed earlier this month that “none of the stations could have been ready for December” and admitted there were “still thousands of hours of construction work to do in the tunnels”.
Testing of the signalling systems is also taking much longer than expected.
Crossrail’s cost is being met by the Government, the Greater London Authority (GLA) including Transport for London (TfL), and London businesses.
Its budget has fluctuated from £15.9 billion in 2007 to £14.8 billion in 2010.
But due to the cost of the delayed opening, a £2 billion Government bailout of loans and cash was announced in December.
KPMG was commissioned to carry out its review into Crossrail Ltd’s governance and finance arrangements by the Department for Transport and TfL.
The auditor made a series of recommendations to control the project’s future costs, including identifying gaps in resources and improving transparency.
A TfL spokesman said: “We are now considering KPMG’s recommendations and are working with Crossrail Ltd to implement them as quickly as possible.
“We and the DfT, as joint sponsor, have also already taken a number of actions to strengthen the governance and assurance on the project. Everyone involved remains fully focused on ensuring the Elizabeth Line is completed as quickly as possible and brought into service for passengers.”
Once fully opened, the Elizabeth Line will run from Reading and Heathrow in the west to Shenfield and Abbey Wood in the east, through 13 miles (21km) of new tunnels in central London.