The planned #72 million metro tram extension through Birmingham city centre is on the brink of collapse.

Council engineers fear the West Midlands Passenger Transport Authority's insistence on running 15 trams an hour will impose impossible demands on busy road junctions, plunging major routes into gridlock.

The business case for the extension from Snow Hill to Edgbaston, which must get Government approval before the project can go ahead, has still not been completed more than a year after a public inquiry backed the scheme.

Council sources made it clear yesterday they would not rubber-stamp the business case until Centro, the passenger transport executive, provided modelling information showing the impact of 15 trams an hour on city centre roads.

Without the data, council engineers cannot work out the impact on traffic flows of the metro extension.

The Birmingham Post has learned of an unusually blunt letter from David Pywell, council strategic director of development, to Rob Donald, the director-general of Centro.

Mr Pywell warned: "We can't keep dancing around this issue. The modelling information needs to be completed and analysed and we need technical solutions (if there are any) that I can recommend to my members. I am prepared to do that but it is not possible at the moment."

Mr Pywell made it clear Centro's decision to switch from ten trams an hour to 15, announced in January, would impact on other road users.

He added: "The business case assumes 15 trams an hour to Edgbaston. We have been waiting for weeks for the modelling work to decide how this can be coped with, if at all, and we haven't yet got it."

A council insider close to negotiations with Centro said the metro extension was now in a perilous position. Only by having 15 trams an hour could Centro meet the Government's cost-benefit criteria for public transport projects, but the impact on traffic flows would be huge.

Trams would cross over junctions once every two minutes rather than once every three minutes, with traffic lights being used to give the trams priority. This would cause difficulties at several junctions, particularly Broad Street and Paradise Circus.

Centro is insisting the business case remains on course for approval and that the tram system will lead to a reduction in the number of people driving into Birmingham.

A spokesman said: "We are confident that further modelling work being completed by council officers will be sufficient to submit an approved plan to the Government.

"It is our firm belief that 15 trams an hour is the best solution for passengers and that modern trams running every few minutes will do the most to reduce congestion in the city centre."

The council's Conservative-Liberal Democrat coalition, which had wanted an underground train system, is also concerned at the "consequential costs" of the metro extension in terms of improving junctions and widening roads.

Councillor Len Gregory (Con Billesley), cabinet member for transportation, admitted it was possible the extension might not go ahead.

"We are waiting for the data from Centro and until we get that we cannot take an objective view of whether this scheme is value for money. We have to look at the wider impact on junctions close to the route.

"If you are having trams crossing junctions every two minutes then that is clearly going to have a major effect on a road network that is already facing serious congestion difficulties."

The metro business case was to have been approved by the cabinet before Christmas. It is now due to be discussed at the end of next month.