The Conservatives have denied breaking a manifesto pledge after their shadow minister for London refused to guarantee the future of Crossrail.
Hundreds of millions have already been spent on the £16bn project to link the east and west of London by rail.
But MP candidate Justine Greening told LBC radio that, although they supported it, they could offer no guarantee.
Both Labour and the Liberal Democrats condemned the stance towards a "vital project" for Londoners.
Crossrail will link towns to the east and west of the city with Heathrow, the West End and Canary Wharf.
The project's supporters say it will bring better transport for London and create jobs when they are badly needed.
The Conservatives manifesto said the party supported "Crossrail and the electrification of the Great Western line to south Wales" and a party spokesman said that had not changed.
But Ms Greening said: "We've always said that we think it's an important project and actually the reason this is important is we want to be responsible, so we can't pretend that we can write an entire budget outside of government.
"We've said we'll do one within 50 days of getting into government if we get elected and we will then provide some clarity and certainty."
Tessa Jowell, Labour Minister for the Olympics in the previous Parliament, said: "The Conservatives are offering weasel words on Crossrail.
"We are offering an unequivocal guarantee to go ahead with construction."
And Lib Dem candidate David Laws said he was "shocked" at the Conservative Party's stance.
He added: "They are looking at cutting a really vital project for London."
The Tory spokesman said the only two departments whose budgets they would guarantee not to cut were health and the Department for International Development.