Costs associated with building Crossrail are "likely to continue to climb", a watchdog has said.
The Commons Public Accounts Committee said commuters in the South East have been let down by the delay in opening the new rail line.
A report published by the MPs noted that funding for the programme has already increased by around £2.8 billion to £17.6 billion, and "the final cost still remains unknown".
Crossrail was due to be completed in December last year, but the railway may not open in full until 2022.
The project is being jointly funded by the Department for Transport and Transport for London.
It is being built by Crossrail Ltd, which is an arm's-length body owned by TfL. The PAC found that Crossrail Ltd has "so far failed to understand the complexity and risks involved in the programme".
It added that the DfT "still does not appear to have got a grip" on the problem of cost increases and delays on major rail projects, citing the modernisation of the Thameslink and Great Western routes as previous examples.