London Mayor Boris Johnson has raised doubts over the £16bn Crossrail project going ahead "in its entirety".
He said branches to Abbey Wood, in south-east London, and Maidenhead, in Berkshire, could be threatened under the new government.
Transport secretary Philip Hammond has already said the project may not be completed by 2017.
Work has been under way since January 2009 on the route running from Berkshire to Essex across London.
The 72-mile (116km) rail line will connect Maidenhead with Shenfield in Essex passing through the West End and Canary Wharf. There will also be a link to Heathrow Airport.
Speaking at City Hall, Mr Johnson said he could come under pressure from the new government to modify the scheme and do away with the spurs.
"They would try to de-scope. I would be fighting very very hard to protect the project in its entirety. It's absolutely vital that we mount a Stalingrad-like defence of the London transport settlement.
"It's fascinating, the degree of ignorance about Crossrail still in the minds of the public and indeed in the minds of many of our important political colleagues.
"The more we can explain why it matters to London, the better."
In the run up to the elections, the Conservatives had come under fire when they refused to guarantee the funding of the project.
But on Friday Mr Hammond pledged his commitment to the project but warned it was essential to keep costs as low as possible although he said he could not guarantee the route would be ready on time by 2017.
The mayor said delaying the completion will add to the costs and not reduce it in the long-term.
Hotel groups have warned the mayor that his plans to make them pay an extra £66 per sq m if new hotels are built near the route could harm the tourism industry.
The Mayor's office said its impact on tourism should be minimal, adding that for an initial three-year period there will be a 20% reduction in the charge on new developments.