WASHINGTON — The nation’s leading antitrust enforcer said this week that it would be difficult for the Justice Department to approve a merger among any of the top four wireless phone companies, casting doubt on recent speculation that T-Mobile and Sprint might consummate a deal in coming months.

William J. Baer, assistant attorney general for the antitrust division, said in an interview that further consolidation among the top wireless carriers would face intense scrutiny because consumers have enjoyed “much more favorable competitive conditions” since the division blocked a proposed merger between AT&T and T-Mobile in 2011.

“It’s going to be hard for someone to make a persuasive case that reducing four firms to three is actually going to improve competition for the benefit of American consumers,” he said, without referring to any specific merger proposal.

Baer said that the division would similarly scrutinize any proposed merger among cable television companies. Analysts say that the cable market has evolved in recent years from a largely local market to a national one, where advertising, programming and sometimes subscription rates are set nationally.

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Baer is expected to warn a group of antitrust lawyers on Thursday that the antitrust department too often sees merger proposals that include little more than token efforts to deal with competitive issues.

Those fig leaf offers are often an attempt to disguise what is essentially an effort to eliminate a big-market participant, Baer is expected to say, while giving up something to a tiny rival that, in truth, does not play a key role in industry competition.