Comcast CEO Brian Roberts speaks at his keynote address at the Consumer Electronics Show in Las Vegas, Jan. 8, 2008. Reuters/Rick Wilking

(Reuters) -- Comcast (NASDAQ:CMCSA) may divest itself of about 4 million subscribers after its merger with Time Warner Cable (NYSE:TWC) and is working on a deal to sell a portion of those customers to Charter Communications (NASDAQ:CHTR), a source familiar with the matter said Tuesday.

Comcast is studying what could be a two-phase process, where Charter would first acquire up to 1.5 million subscribers directly from Comcast, the source told Reuters on condition of anonymity because discussions were continuing and a final agreement had not been struck.

Comcast would then spin off the rest of the 2.5 million subscribers into a separate, publicly traded company, which Charter would take a minority stake in, the source added.

The roughly 4 million subscribers are worth between $18 billion and $20 billion, said the source.

Comcast and Charter would also swap about 1 million cable subscribers, including Charter's subscribers in Los Angeles. It was not yet clear what markets Comcast would swap with Charter or spin off, although the companies probably would trade subscribers adjacent to their existing markets.

Any agreement the companies reach would be contingent on Comcast's $45.3 billion acquisition of Time Warner Cable being approved by regulators, a process that could take many more months while the Justice Department and U.S. Federal Communications Commission study that deal.

While the source characterized the negotiations as serious, the deal could still fall apart and other cable companies and private equity firms may be interested in the divestitures. Representatives for Comcast and Charter declined to comment.

The Wall Street Journal and Bloomberg both reported parts of the deal earlier on Tuesday.

Comcast and Charter originally teamed up last year to try to find a way to carve up Time Warner Cable. But talks soured as the two bickered over price and the feasibility of engineering a split of the No. 2 U.S. cable operator.

Charter, backed by billionaire John Malone's Liberty Media Corp, had pursued Time Warner Cable for months before Comcast swooped in with a surprise bid.

Comcast's chief financial officer, Michael Angelakis, said on Tuesday there was no timeline governing when the company will make a decision regarding the divestitures.

Speaking on a conference call following Comcast's better-than-expected earnings Tuesday morning, the CFO said the company was studying a "number of potential structures" and wants to pursue divestitures in the "most tax-efficient way possible.

Any cash proceeds from a spinoff or sale would be used for returning capital to shareholders, he said.

(Reporting by Liana B. Baker; Editing by Bernard Orr)