A coalition of leading open internet advocates is pressuring federal regulators to crack down on the controversial broadband industry practice of "zero-rating," calling it a threat to net neutrality, the principle that all content on the internet should be equally accessible.

Zero-rating refers to a variety of practices that broadband companies use to exempt certain internet content and services from data caps, effectively favoring those services by giving consumers an economic incentive to use them instead of rival offerings.

For example, AT&T offers a "sponsored data plan" that allows wireless customers to access certain services that don't count against monthly data limits. Similarly, T-Mobile's "Binge On" plan exempts some video services from data caps, while counting others against the monthly limits.

On Tuesday, more than 50 firms and public interest groups sent the FCC a letter warning that zero-rating poses a serious threat to internet openness. The coalition called for the agency to launch a transparent process so that industry and nonprofit stakeholders can weigh in on the matter.

"Zero-­rating profoundly affects internet users' choices," the groups wrote. "Giving ISPs the power to favor some sites or services over others would let ISPs pick winners and losers online—precisely what the open internet rules exist to prevent."

The letter's signatories include Medium, Mozilla, Kickstarter, and the powerful New York-based venture capital firm Union Square Ventures. The Center for Media Justice, Fight for the Future, Demand Progress, and Access Now also signed.

To the extent that zero-rating harms internet openness, the effect of those harms will be felt most acutely by poor people

The tech groups called on the FCC to open a transparent review process so that the public and industry and nonprofit stakeholders can have a chance to help inform the agency's policy.

Zero-rating has become the latest battlefront in the decade-long war over how best to safeguard the internet's open nature. In its landmark 2015 net neutrality policy, the FCC did not explicitly prohibit zero-rating, but rather indicated it would address the issue on a case-by-case basis.

The nation's largest cable and phone companies loathe the FCC's net neutrality rules—arguing that they stifle investment and innovation—and have challenged them in a federal court. A ruling on that case could come as early as this week.

But even as the industry giants pursue legal action, they've forged ahead with zero-rated plans.

Wireless companies like AT&T and Sprint, in particular, have pushed ahead aggressively with zero-rating, which they frame as a "gift" to consumers that allows them to use certain services without exceeding data caps.

Open internet advocates argue that zero-rated wireless plans disproportionately affect low-income communities and communities of color, who are more likely to rely on mobile networks to access the internet. In other words, to the extent that zero-rating harms internet openness, the effect of those harms will be felt most acutely by poor people.

"Poor people, communities of color, and people who have been pushed to the margins of society need equal and affordable access to the whole internet, not just these companies' preferred portions of it," Malkia Cyril, co-founder and director of the Center for Media Justice, told Motherboard in a recent interview.