The government wants the private sector to foot the major part of the bill for Crossrail, which could be as much as �15bn.

It was hoped that it would be ready in time to carry people to the 2012 Olympics, should London be successful in its bid to host the games.

But Transport Secretary Alistair Darling has now said that will be unlikely.

On Sunday, Mr Darling told BBC's Breakfast With Frost: "It was never going to be in place for the Olympics, we have made that clear from the start when we decided to put in a bid."

He believes London mayor Ken Livingstone was too optimistic in suggesting the project to link Heathrow with Stratford could be ready by 2012.

It is a huge project, one of the biggest the country will see Alistair Darling

"Crossrail is about building a new east-west rail link through a tunnel through central London - now in any view that is going to take some time to do," he said.

"We believe it is essential for London's expansion, the big question is how is it going to be paid for?"

He said the next stage would be to consult on the route and to ask for a "substantial contribution" from the private sector.

"We are talking about a project that could be between �10bn and �15bn in cost. It is a huge project, one of the biggest the country will see."

Vital regeneration

Last month Mr Livingstone and five council leaders wrote to Mr Darling to ask that he make a decision before Parliament rises on 17 July.

They are concerned if a decision is not made before then, the timetable for the whole project will be thrown off course.

A leaked Treasury report suggested the multi-billion pound project was an essential part of London's bid to stage the 2012 Olympics.

And supporters say the multi-billion pound project is a vital part of the regeneration of east London.