Two former Treasury secretaries joined two former Federal Reserve leaders on Monday to warn that the Trump administration’s efforts to relax oversight of certain financial firms could seriously threaten the stability of America’s financial system.

The stark warning came two months after a federal oversight panel said it planned to stop designating large, non-bank financial institutions like insurers and asset managers as “systemically important” and placing them under stricter federal oversight.

The Financial Stability Oversight Council move would ease a process put in place after the 2008 financial crisis that aimed to prevent non-bank financial firms, like American International Group, from posing a risk to the American economy. Instead of designating companies that the government believes pose a risk to the financial system, the oversight council has proposed designated “activities” that pose a risk, such as certain financial products.

Regulators who dealt with the aftermath of the crisis fear that dropping the designation could be a grave mistake. On Monday, the former Treasury secretaries, Jacob J. Lew and Timothy F. Geithner, and the two former Federal leaders, Ben S. Bernanke and Janet L. Yellen, urged their successors, Steven Mnuchin, the Treasury secretary, and Jerome H. Powell, the Fed chairman, to rethink the plan.