All four major service providers in the U.S. have been accused of violating the last remaining net neutrality rule, a day after President Barack Obama said he was against the Federal Communication Commission’s proposal to institute controversial internet “fast lanes” that would further threaten equality for web-based companies.

T-Mobile, Sprint, AT&T, and Verizon have all been accused of violating the transparency rule left in place to help safeguard net neutrality after a federal appeals court ruling knocked down several anti-discrimination and anti-blocking regulations.

The transparency requirement was upheld to ensure that carriers disclose their information about network performance and management policies, but consumer watchdog group Public Knowledge on Wednesday sent letters to the companies accusing them of shrouding their throttling plans in secrecy and demanding that they disclose or suspend the practices.

Throttling is the intentional slowing down or limiting of customers’ internet service by a provider — a controversial practice that has ended in court for cable companies like Comcast.

“Sprint and Verizon violate the transparency rule by failing to meaningfully disclose which subscribers will be eligible for throttling,” Public Knowledge wrote. “AT&T, Sprint, and Verizon violate the transparency rule by failing to disclose which areas of the network are congested, thus subject to throttling. T-Mobile violates the transparency rule by preventing throttled subscribers from determining the actual network speed available to them.”

The letters come as the FCC continues a long-standing debate over whether to abolish net neutrality entirely and instead establish a discriminatory “paid prioritization” model that would create fast and slow internet lanes, allowing wealthy companies to simply pay ISPs for speedier services, while those without as much capital would be forced to contend with sluggish or unreliable connections and loading times. The model was introduced by FCC chairman Tom Wheeler, a former lobbyist for the cable and wireless industries. While supporters say the proposal would not lead to “commercially unreasonable” arrangements with big companies, numerous advocacy and internet freedom organizations have opposed and disputed it, noting that it would risk development opportunities for new startups and allow providers to charge customers more for services.

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At the US Africa Leaders Summit on Wednesday, Obama voiced his disagreement with the FCC proposal, saying, “you don’t want to start getting a differentiation in how accessible the Internet is to different users. You want to leave it open so the next Google and the next Facebook can succeed.”

Craig Aaron, president of internet watchdog group Free Press, said the FCC’s plan “would encourage individualized negotiations and allow new kinds of discrimination. It would strand the next Google or Facebook in the slow lane.”