The City of London Corporation has agreed to meet a funding gap holding back London's £16bn Crossrail.
The Prime Minister urged the City to make a "significant contribution"
The authority voted to support "a financial contribution" to the project, which had stalled over a shortfall of up to £400m.
Corporation sources told BBC London they were confident the project's last hurdle had now been overcome.
The planned rail link would connect Maidenhead in the west to Essex, through the centre of the capital.
The corporation's vote follows the government's signal it would provide cash, believed to cover a third of the overall costs, for Crossrail.
'Construction in 2010'
The rest of the money would be made up from borrowing against future fares income, and a levy on London business rates.
The scheme had stalled over the cash shortage, with Prime Minister Gordon Brown urging the City of London to make a "significant contribution".
The City of London Corporation said it had agreed further funding but would not comment on whether that bridged the financial gap in full.
"At a special meeting held this afternoon at the Guildhall, the City of London's Court of Common Council voted, after lengthy discussion, to support a financial contribution to Crossrail," it said in a statement.
Mayor of London Ken Livingstone welcomed the corporation's decision to contribute towards funding Crossrail.
He said: "It reflects the benefits it will bring to the City of London and indeed to London as a whole - other London businesses such as the Canary Wharf Group and BAA have also contributed, as well as Transport for London.
"I hope it will now be possible to agree a final overall funding package for Crossrail and for a positive announcement to be made."
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.
If the project gets the final go-ahead, construction would begin in 2010 with trains up and running by 2015.