Transport Secretary Philip Hammond has committed the Government to delivering the full £16bn Crossrail scheme.
Mr Hammond made the announcement at the site in east London's Docklands where work began on the Canary Wharf Crossrail station in May 2009.
Transport Minister Theresa Villiers has said she will look for "value for money" and that the transport budget was not ring-fenced from cuts.
But Mr Hammond said: "The Government is committed to this project."
He added: "We have no plans to reduce its scope. We want this project to be delivered in its entirety," he said.
But he added that the Government wanted to ensure that Crossrail was delivered "with maximum value for the taxpayer".
The 72-mile (116km) route, due to begin service by 2017, will connect Maidenhead, Berkshire, with Shenfield in Essex via the West End and Canary Wharf, with a link to Heathrow Airport.
RMT general secretary Bob Crow said: "The Government have sent out the clearest possible signal that transport is going to be right in the front line as the cuts assault on our services is unleashed."
He added that Crossrail was "on the critical list as the ConDems look to dump manifesto commitments to vital transport modernisation projects."