Crossrail is more important for London's future than the Olympics, Ken Livingstone announced as he launched a campaign for the high-speed rail link.
The rail link would cut journey times from Heathrow into London
Supported by business and trade union leaders, the mayor said Crossrail was "the transport system vital to underpin London's 21st Century economic engine".
He said key decisions to enable its construction must be taken while the Bill is going through Parliament.
Critics of the £15bn scheme have said it is too big to build and too costly.
The Commons select committee is due to hear evidence on the scheme next week.
Speaking at City Hall on Tuesday, Mr Livingstone said he was launching the campaign because "Crossrail is so vital for the income and jobs of Londoners".
"They (business and trade union leaders) understand it is vital not only for London but for the UK's economy and businesses.
"Crossrail is not only a national priority. It is even more important to London's long-term prosperity than our victory in winning the Olympic Games."
A number of events designed to demonstrate the support for the scheme are planned as part of the campaign.
Funding has yet to be secured for the scheme which would stretch from Berkshire to Essex through tunnels beneath London.