New transport secretary Philip Hammond has pledged his commitment to the Crossrail scheme under the new coalition government.
Work has been under way since January 2009 on the £16bn rail project, which will run through London connecting Maidenhead in Berkshire with Essex.
Mr Hammond said he was committed to the project but warned it was essential to keep costs as low as possible.
But he could not guarantee the route would be ready on time by 2017.
The 72-mile (116km) scheme will include a link to Heathrow airport and will run through central London and out eastwards to Shenfield in Essex.
The announcement was welcomed by business leaders in Maidenhead.
Mike Miller, president of Maidenhead Chamber of Commerce, said: "Maidenhead is a commuter town and this will make it much easier to get to London with one train.
"The town will become much more important throughout the south east.
"It will also mean we will have greater access for consumers to come here from London and other areas, but we must be attractive and we are looking at what needs to improve."
The Department for Transport said the new government was very supportive of the scheme.
A spokesman said: "It is a very important project for London, Essex and Berkshire.
"We need to ensure every pound invested in the project is well spent.
"Going forward we are going to explore how we can get the utmost value."