Fourteen months after unveiling a $45.2 billion merger that would create a new Internet and cable giant, Comcast Corp. is planning to walk away from its proposed takeover of Time Warner Cable Inc., people with knowledge of the matter said.

The decision marks a swift unraveling of a deal that awaited federal approval for more than a year. Opposition from the U.S. Justice Department and Federal Communications Commission took shape over the past week, leaving officials of the two companies to conclude the deal wouldn’t pass muster.

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