The government has given the go-ahead for the £10bn Crossrail project that will link east and west London.
But the Department for Transport has again stressed that the west-east link was "very, very unlikely" to be ready in time for the 2012 Olympic Games, for which the city is bidding.
Transport Secretary Alistair Darling said he fully supported the scheme in principle and that he would be consulting with the Treasury to see how much taxpayers' money would be going into the project.
Mr Darling is also assembling a team to look at the proposal for the link, which will speed train journey times through London.
The scheme involves a central east-west tunnel with services extending to two branches to the east and two to the west.
This would involve a central tunnel from west of Paddington to east of Liverpool Street
From Liverpool Street, the line would extend through Whitechapel, then dividing into two branches both serving the Thames Gateway - one going to Stratford and joining the existing Great Eastern lines to Shenfield
The other will go down to the Isle of Dogs and the Royal Docks, crossing the Thames to join the lines at Abbey Wood, with some services continuing to Ebbsfleet in Kent
To the west of Paddington, the line would join the Great Western main line before dividing into two further branches
One would join up with the North London Line, providing services to Richmond and Kingston
The other would be via Ealing and Hayes to Heathrow along the existing branch served by Heathrow Express
Crossrail could carry 200,000 people during the morning peak period and it is thought could create up to 100,000 jobs.
An earlier attempt in the 1990s to get the Crossrail project through Parliament failed.
But the present government allocated £154m to carry out a feasibility study and the project is being taken forward by Cross London Rail Links - a joint venture company formed by Transport for London (TfL) and the Strategic Rail Authority (SRA).
In a House of Commons written ministerial statement, Mr Darling said that if the project was to go ahead there would need to be "a very substantial contribution to its costs" from London's business community".
Crossrail line 1, the first part of the scheme, will create a new network of services linking areas as far east as Shenfield in Essex, to Reading in Berkshire in the west.
The heart of the project is the construction of a new tunnelled route across London, with new stations at Liverpool Street, Farringdon, Tottenham Court Road, Bond Street and Paddington.
It also includes the option to include Heathrow in its service.
Crossrail line 2 would create a network of services linking Clapham Junction in the south-west to Dalston in the north-east.
It would mean tunnelling between Victoria and King's Cross with an interchange with Crossrail line 1 at Tottenham Court Road.