Previously the government was set to pay £350m towards the cost of the rolling stock and in September 2012 invited prospective bidders to apply for a government guarantee to assist in the financing of the new trains.

But under changes put forward by London Mayor Mr Boris Johnson, and agreed by transport minister Mr Patrick McLoughlin, the public sector will now pick up all of the costs. With no existing trains capable of operating through the tunnels, Transport for London says the aim of the new funding structure is to ensure timely delivery of the trains ahead of the commencement of operations at the end of 2018.

The government aims to have a deal in place for the new trains by 2014, with delivery and testing starting in 2017.

"It is fantastic news that we can crack on with buying the wonderful new fleet of brand spanking new trains," Johnson says. "With more than 600 coaches providing a 10% increase in London's rail capacity, Crossrail will transport not just passengers but jobs and growth across the city and beyond."