AT&T has reached a deal to buy Time Warner Inc. for $85.4 billion — a blockbuster marriage that would transform the telephone company into the nation’s largest entertainment company and a major force in Hollywood.

The agreement, which was hammered out at breakneck speed and announced by the two companies Saturday afternoon, accelerates the wave of consolidation sweeping through the telecommunications and media industries.

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Mark Lemley, a professor at Stanford Law School and director of the Stanford Program in Law, Science and Technology, said Saturday that the merger raises “very significant” antitrust concerns.

“I would be surprised if the antitrust authorities let this one pass,” Lemley said. “There has been a wave of big mergers by direct competitors, and the government has been increasingly aggressive in challenging those mergers.”