On Wednesday he was urged to back the Crossrail project amid reports it faced the axe.

During question time at the Commons, he said it was "well worth supporting" and would benefit the whole country.

But he warned Simon Hughes, the Liberal Democrat's London mayoral candidate, that work needed to be done to ensure the scheme could be properly funded.

"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," Mr Blair said.

"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.