Gordon Brown signalled the green light for Crossrail this week after saying the autumn's Comprehensive Spending Review will 'make room' for the £16 billion job.
Mr Brown told an audience at Canary Wharf: "The government will make cash available for the £16bn Crossrail project in this month's Comprehensive Spending Review."
But the cash is only available if "only if others are prepared to play their part," he said, pointing out that the City would be expected to contribute towards the cost.
In June, Mr Brown signalled his support for the project in a speech to the City. He said: "I recognise the benefits Crossrail would bring to the City we are using every effort to find a solution to its affordability.
"I will ensure this work is stepped up but as you know the only financing solution that will work will require all parties - public and private - contributing significantly."