The prime minister has told MPs he is "confident" that a £10bn scheme to build an east to west rail link underneath London will proceed.
Urged to back the Crossrail project amid reports it faced the axe, he said on Wednesday it was "well worth supporting" and would benefit the whole country.
But, at Commons question time, he warned Liberal Democrat London mayoral candidate Simon Hughes that work was needed to ensure it could be properly funded.
He said: "We remain convinced that Crossrail is an important project for London, that it is well worth supporting, that it could have real benefits for Londoners and indeed for the whole country.
"It is important however, as government, that we sit down and work out how that can be funded because as you know it runs into many billions of pounds.
"But we are confident that if we sit down and look at this in the right way that we can take it forward."
Mr Hughes had called on the prime minister to give the scheme "a personal vote of confidence" so the private sector could know it was worth investing in.