The prime minister has said the government will provide funding for London's Crossrail project - but only if businesses provide money as well.
The route would link firms in the City of London to Heathrow
Gordon Brown said financial services companies in the City financial district would have to make a "significant contribution".
The planned rail link would connect Maidenhead in the west to Essex, through the centre of the capital.
Mr Brown did not say how much money he would commit to the £16bn overall cost.
"No government has previously managed to bring the necessary finance together, in the last few months we have worked night and day to achieve this," he said.
The private sector's contribution would be achieved through a levy on businesses that would benefit from the project.
At the Conservative Party conference, the Shadow Chancellor, George Osborne, also gave his backing to Crossrail.
He said a Tory government would "get people and goods moving freely on uncongested roads and high speed railways, and yes Crossrail".
Crossrail was first proposed in the 1980s but supporters have had difficulty in securing the funding.
As well as adding capacity to London's overcrowded tube network, it would link the capital with its airports.
The final go-ahead for the project could come as soon as the government's comprehensive spending review later this month.