The issue was raised this week in a new publication by a pivotal member of the steering group charged with overseeing what is a key scheme for the region.
Group member Centro, a passenger transport organisation, has identified £10 million funding-gap 'problems'.
This concern has arisen because the required money was not transferred from the Strategic Rail Authority (SRA) to the steering group before the SRA was wound up in August.
This funding should have been transferred to the Department of Transport, which sits on the steering group along with Centro, Network Rail, Birmingham City Council and Advantage West Midlands.
Centro has said it will not commit to further design work until the other group members commit to filling the funding gap necessary to take the project to its implementation stage.
According to Centro, the issue will be addressed at a committee meeting held by the Passenger Transport Authority on Monday 5 September.
A spokesperson for the city council rejected the idea that there is any cause for concern. He said: 'This is a very important project for the city, and we are confident that all the partners concerned in the plans for New St Station will make sure that the funding is in place for the scheme to go forward.'