It's no secret that big cable companies don't like to compete against each other, as it's more profitable to be the only company in town than to build networks in places already dominated by another cable provider.

But for Comcast and Charter, the two biggest cable companies in the US, that aversion to competition is going to extend beyond cable networks and into the mobile market. The companies today announced an agreement to cooperate in their plans to sell mobile phone service, an agreement that also forbids each company from making wireless mergers and acquisitions without the other's consent for one year.

"That agreement could stoke Wall Street speculation among investors and analysts that the two largest US cable companies together could decide to make a play for a carrier like T-Mobile US Inc. or Sprint Corp.," wrote The Wall Street Journal, which reported the Comcast/Charter agreement hours before it was announced. "Neither company as a single entity could buy another wireless carrier for that time period as a result of that agreement without the other’s blessing or involvement."

The deal could violate antitrust law, said Harold Feld, an attorney and senior VP of consumer advocacy group Public Knowledge.

"One of the basic ideas of antitrust law is that when companies that compete with each other, or could compete with each other, make an explicit agreement to not compete with each other, that violates the antitrust laws," Feld told Ars today. "Agreeing to coordinate with each other to avoid competition is expressly a violation of the antitrust laws."

But that doesn't mean Comcast and Charter won't be able to follow through with their plan. It's impossible to say with absolute certainty whether any specific agreement violates antitrust law, and "both Comcast and Charter have very good lawyers," Feld said.

Antitrust enforcement also depends heavily on choices made by the White House. President Donald Trump has appointed a new antitrust chief whose views "contrast sharply" with the Obama administration's "more aggressive approach in antitrust toward protecting innovation, especially in the technology industry," The New York Times recently wrote.

Comcast and Charter have a combined 47 million Internet subscribers, dominating the US market for high-speed broadband, but they do not compete against each other in any city or town. The Comcast/Charter cooperation agreement fits in nicely with Comcast's mobile plans, because the company intends to sell smartphone data plans only to customers who also have Comcast home Internet service. Comcast's mobile service is scheduled to be available by the end of June, while Charter has said it intends to offer similar service in 2018.

Comcast and Charter "are each regional cable companies that have from time to time cooperated on business strategies and initiatives in an effort to better compete against our mutual competitors," the companies said today. "We each have planned or have nascent regional wireless operations that are currently limited to our respective cable distribution footprints."

Public Knowledge issued a statement saying that the Comcast/Charter deal "is another demonstration of the cable business model: regional monopolies that collaborate with each other and stay out of each other’s markets. A deal like this simply formalizes what everyone already knows: that Charter and Comcast have no intention of ever competing with each other."

We asked Comcast and Charter about the antitrust implications of their deal today. Charter responded, "We believe this agreement is pro-competition. As two regional operators, this agreement is about exploring potential opportunities to find efficiencies, accelerate our launch and enhance our ability to participate in the national wireless marketplace. We don’t compete with Comcast."

We will update this story if Comcast provides a response.

UPDATE: Comcast responded to us and said, "the Comcast-Charter agreement does not violate the antitrust laws. Comcast and Charter are not competitors or even potential competitors in the wireless market. Our agreement to cooperate will help increase our competitive viability in a market controlled by national players and, therefore, promote competition and consumer choice.”

Cable firms to resell Verizon mobile service

Comcast and Charter each have agreements with Verizon Wireless to resell that company's mobile service under their own brands. The Comcast/Charter announcement said the cable companies will "explore working together in a number of potential operational areas in the wireless space, including: creating common operating platforms; technical standards development and harmonization; device forward and reverse logistics; and emerging wireless technology platforms."

Additionally, Comcast and Charter said they "have agreed to work only together with respect to national mobile network operators, through potential commercial arrangements, including MVNOs [mobile virtual network operators] and other material transactions in the wireless industry, for a period of one year."

The "material transactions" that require both companies' consent include "investments, acquisitions, mergers, partnerships, joint ventures or similar transactions" worth at least $50 million, according to a filing with the Securities and Exchange Commission. The companies are not allowed to discuss any such transactions with third parties without consent during the year of the agreement, even if the actual transaction would take place after the year is up.

The cable companies' reseller agreements with Verizon stem from a 2012 transaction in which Verizon purchased spectrum from a consortium of cable companies. That agreement also let Verizon resell its competitors' cable products and let the cable companies resell Verizon Wireless service.

The Federal Communications Commission and Department of Justice approved that deal, but only after forbidding Verizon from reselling cable products and services in places where Verizon FiOS Internet and TV service already exists or where Verizon is authorized or obligated to deploy FiOS.

The anti-competitive provisions that the government struck out of the original Verizon/cable deal "were very similar" to the ones described in today's Comcast/Charter announcement, Feld said.

Disclosure: The Advance/Newhouse Partnership, which owns about 13 percent of Charter, is part of Advance Publications. Advance Publications owns Condé Nast, which owns Ars Technica.