Strathclyde Partnership for Transport (SPT) has raised formal concerns over the private scheme – which is controversially backed by millions from Glasgow City Council – amid concerns it will create traffic gridlock in the city.
Experts are particularly worried about the future supermall's car park, which will have a single entrance on the already congested North Hanover Street, and could disrupt vital bus routes.
The expanded mall will have 1300 fewer parking spaces than the current site's 3000 places.
SPT has issued a holding objection to an application for planning permission at Buchanan Galleries.
This is despite plans by Glasgow City Council, one of SPT's core partners, to borrow over £80m to support the expanded shopping centre as part of a controversial tax incremental funding or TIF scheme.
An SPT spokeswoman said: "It is our job to make sure that access to public transport is key to any development plans.
"SPT's initial feedback on this application is based on our concerns over the impact of increased traffic from the car park design entrance and exit points.
"As it stands, it would undoubtedly cause disruption to bus services, particularly on North Hanover Street, and is not allowing enough flexibility for longer term services – such as the proposed Fastlink."
SPT, The Evening Times can reveal, is also worried about the impact on bus movements at its Buchanan Street hub.
The Millennium Hotel, in George Square, has also lodged an objection to the plans, citing concerns about traffic .
The owners of Gasgow's St Enoch Centre, Ivanhoe Cambridge, is a long-standing opponent of the TIF plan, which it believes amounts to unfair state support of one business against another.
It also has concerns over traffic.
Nick Davis, development director at Buchanan Galleries, today insisted the bigger mall would be good for traffic.
He said: "One of the principles of the proposed development is to provide improved access which will include improved, safer pedestrian linkages from Buchanan bus station and surrounding streets to Queen Street Station and George Square."
Glasgow City Council has still to consider planning permission for the scheme.