Critics say the money could be better spent on the London Underground
Prime Minister Gordon Brown has marked the start of construction on a new £15.9bn rail route across London.
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.
But Crossrail has been criticised by some commentators as a waste of money, given the extreme pressure on the public purse.
Given the go-ahead in 2007, Crossrail will involve digging tunnels from the east of London to a point close to Paddington station in the west of the city.
Boris Johnson says Crossrail is crucial to London's future prosperity
The tunnels will link to existing rail stations, providing train services to Maidenhead, Heathrow, central London, Docklands, Shenfield in Essex and Abbey Wood in south London.
Mr Brown joined rail minister Lord Adonis and London Mayor Boris Johnson at Canary Wharf in London Docklands as a foundation for a new Crossrail station was laid.
Mr Brown said: "Investment in important projects like Crossrail, the largest construction project in Europe, is vital to create and protect jobs as well as supporting business, so we can grow our way out of recession and ensure a strong future for London and the country as a whole."
There have been concerns that the Conservatives could abandon the project because of the economic conditions should the party win the next general election.
BBC transport correspondent Tom Symonds said: "Critics say it's far too much when public spending is expected to fall and some say the money would be better invested in the tube system."
See details of Crossrail route through central London
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