With ridership still far below pre-pandemic levels, the Massachusetts Bay Transportation Authority faces a calamitous revenue shortfall in years to come that would force fare hikes or service cuts and would make it impossible for the agency to enact the sweeping social and environmental changes advocates demand, a new report from a top fiscal watchdog warned Thursday.

The business-backed Massachusetts Taxpayers Foundation found that the MBTA is on track to face a shortfall as large as $400 million in its yearly budget that begins in 2023 and a $13 billion shortfall for core infrastructure improvements it plans to make over the next decade.