The Federal Trade Commission, which under the Biden administration has been highly skeptical of mergers, has sent shock waves through the dealmaking community by recently using a long-ignored provision of a federal antitrust law to force drastic changes to a $5.2 billion natural-gas-industry deal.

The agency flexed its regulatory muscle by citing the Clayton Act’s Section 8, which bars competing companies from having a person serve as a director of both, an arrangement known as an “interlocking directorate.” It was the first time in 40 years the FTC had invoked Section 8.