"In my view, when the division concludes that a merger is likely to lessen competition or tend to create a monopoly, in most situations, we should seek a simple injunction to block the transaction," said Kanter, who was confirmed in November and is one of three progressives named to top U.S. antitrust posts.

Kanter took the reins of the division at a time of rising inflation, including in key industries like meat production, and growing concern that there are a range of sectors of the economy where a handful of companies have become too powerful.

Kanter argued that asset sales, or divestitures, can fail because the company that buys the assets fails to make full use of them. He also noted that it was often difficult to craft behavioral remedies, for example creating a firewall separating two sectors of a company.

"None of this is to say that divestitures should never be an option. Sometimes business units are sufficiently discrete and complete that disentangling them from a parent company in a non-dynamic market is a straightforward exercise where divestiture might have a high degree of certainty of success," said Kanter. "But in my view, those circumstances are the exception not the rule."

(Reporting by Diane Bartz)