Public interest groups are urging the Federal Communications Commission to take action under its net neutrality rules against the increasing number of Internet providers who allow customers to access some services without charging them for the data.

“As currently offered, these plans enable ISPs to pick winners and losers online or create new tolls for websites and applications,” said public interest advocacy groups in a joint letter to FCC Chairman Tom Wheeler

{mosads}“As a result, they present a serious threat to the Open Internet: they distort competition, thwart innovation, threaten free speech, and restrict consumer choice — all harms the rules were meant to prevent.”

The letter was signed by more than 40 groups, including Demand Progress, the Electronic Frontier Foundation and Free Press.

They are responding to the rise of the practice, known as zero-rating, that allows customers to consume data free of charge, broadly speaking, as long as they use certain services or websites.

The programs vary widely in implementation. T-Mobile’s Binge On allows customers to view content on some video websites without docking their data allotment. Other companies allow firms to sponsor data use when a customer is accessing a certain service.

Critics contend that the practices could violate rules the FCC approved last year to ensure net neutrality, the idea that all traffic on the Internet should be treated in the same way.

“In their current iterations, each of these plans run afoul of both the spirit of net neutrality and of the Open Internet rules,” the groups said. “We urge the FCC to respond to the proliferation of these plans, fulfill its mandate to protect Internet users, and enforce its Open Internet rules.”

The FCC has already started to look into the programs and requested meetings late last year with T-Mobile, AT&T and Comcast, though officials there insist their interest should not been construed as an investigation.