Comcast Corp. is planning to drop its proposed acquisition of Time Warner Cable Inc., according to people familiar with the matter, after stiff resistance from U.S. regulators threw the merger of the cable giants off track.

A withdrawal by Comcast would be a stunning turnaround for the $45.2 billion deal, one of the largest proposed media mergers in years. When it was announced in February 2014, many on Wall Street believed the deal had a strong shot at being approved, albeit with concessions to regulators.

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