Comcast has agreed a $45-billion stock deal to buy Time Warner Cable, a move that would merge the two biggest cable operators in the United States, US media reported.

The boards of both companies have approved the deal and an announcement will be made today, The Wall Street Journal said, although experts caution the acquisition will be subject to a potentially lengthy regulatory review.

If, as expected, the deal is confirmed, the huge transaction would mark Comcast's triumph over the nation's fourth-largest cable operator, Charter Communications and its biggest shareholder, Liberty Media Corp.

Liberty Media, owned by John Malone, is the company behind European cable group UPC.

Comcast, which owns prominent entertainment company NBCUniversal, parent of the NBC broadcast network, is already the dominant force with nearly 22 million video subscribers, compared to Time Warner Cable's estimated 12 million.

The Wall Street Journal cited people familiar with the matter as saying that Time Warner Cable shareholders will receive $158.82 a share in stock for their shares under the deal, about $23 a share above where TWC has been trading. 

Charter has made three offers, its most recent valued at $132.50 a share, which was rejected as too low compared to TWC's goal of $160 a share.

The Journal said TWC had long seen Comcast as a preferred merger partner, saying it approached Comcast about a deal last year in hopes of sidelining Charter.

The New York Times said the merger "may have little impact on consumers."

"Comcast and Time Warner Cable compete in very few markets. As a result, few consumers will see their choices of cable operators reduced," it said.

"Nonetheless, regulators will surely look carefully at the impact on the deal to consumers, and may also focus on whether the combined company will have additional power in negotiations with cable networks, a recent source of tension in the industry."

To appease antitrust regulators, Comcast is expected to say it is willing to divest three million of Time Warner Cable's television subscribers, the Times said.