Brown claimed that the government has worked "day and night" in the last few months to finalise funds. He insisted that further investment must now be made by businesses and developers in the capital.
"I want the project to go ahead, subject to the satisfactory conclusion of detailed talks for further contributions from all the beneficiaries," he said. "The City of London in particular will need to make a significant contribution."
He added that the project is "vital" for the UK's long-term prosperity and pledged to make funding available for the scheme provided that "others are prepared to play their part".
A meeting setting out the future of the proposed rail link between east and west London was being held by the City of London's 125 elected members as Planning went to press.
The government has already staged a series of high-level meetings to finalise finances, including talks between transport secretary Ruth Kelly, communities secretary Hazel Blears and Treasury chief secretary Andy Burnham last month (Planning, 21 September, p1).
A spokeswoman for business lobby group London First said the prime minister's backing bodes well for the scheme.
"He spoke favourably on it and the Treasury is doing everything it can to get a funding deal sorted. We are confident that it will get the go-ahead," she added.