Birmingham's underground railway dream is still on track even though the cost has soared to more than £1 billion, city council leader Mike Whitby has insisted.
He said he was weeks away from publishing details of a comprehensive system, which would incorporate the planned Midland Metro extension from Snow Hill to Five Ways.
The entire venture, consisting of more than one line, would be entirely tunnelled including the city centre metro route. Coun Whitby (Con Harborne) was unable to be specific about the exact cost, although sources suggest the bill to be far more than £1 billion.
It is understood the city council has been told that an entirely subterranean city centre line would require previously agreed metro extensions - to Birmingham International Airport/NEC, on the Hagley Road, and to Perry Barr - to be built concurrently to be financially viable.
It is believed the initial consultants' report costed the overall project at over £2 billion - about £1 billion for the underground city centre line and the same amount for the other lines.
A year ago, during campaigning for the council elections, the Conservatives suggested an underground railway for Birmingham could be delivered for £200 million.
The council leader said he was mid-way through the process of questioning assumptions by consultants Jacobs.
A financial model produced by Jacobs is understood to have concluded that the project would not meet the necessary cost-benefit ratio needed for transportation schemes to qualify for Government support.
Coun Whitby said: "They have been testing the idea of an underground against the Centro model.
"I am challenging the way they have carried it out. I am challenging how robust these comparisons are. We are in the end game."
Financial consultants Deloitte have been appointed to examine the costings and will report shortly.
Coun Whitby said the projected cost had grown because the proposal being brought forward was more comprehensive than the original.
He promised a "strategic option" dovetailing with other rail and bus links in Birmingham.
Coun Whitby said the experience of Lyon in France, one of Birmingham's twin cities, was that while the capital investment required to build an underground was vast, the running costs were far lower than a tram system.
However, the council is realistic about the chances of persuading the Government to support a £1 billion-plus underground system instead of the £72 million metro extension, which already has planning permission.
If the council presses ahead with an underground, a new public inquiry would be required and it is unlikely a final decision could be arrived at for several years.