Google, Netflix and Amazon, along with dozens of other tech firms, are warning that FCC Chairman Tom Wheeler’s proposed net neutrality rules could represent a “grave threat to the open Internet,” adding to a chorus of voices who have come out against the plan.

In a letter sent to Wheeler, the tech firms argue that the rules “would enable phone and cable Internet service providers to discriminate both technically and financially against Internet companies and to impose new tolls on them.”

Wheeler’s proposal comes in response to a ruling by the D.C. Circuit Court of Appeals in January that struck down FCC rules of the road for Internet providers that prohibited them from “unreasonable discrimination” in the treatment of web content. Instead, Wheeler proposed a new standard that would measure whether Internet providers were engaged in “commercially reasonable” practices, potentially allowing for certain types of prioritization of traffic.

“Instead of permitting individualized bargaining and discrimination, the Commission’s rule should protect users and Internet companies on both fixed and mobile platforms against blocking, discrimination and paid prioritization, and should make the market for Internet services more transparent,” the tech companies said in their letter. “The rules should provide certainty to all market participants and keep the costs of regulation low.”

Microsoft, Linked In, Facebook and nearly 150 0ther companies signed on to the letter.

The letter is particularly significant because many of the companies who are signatories also had a role in the unprecedented online protect over the Stop Online Piracy Act in 2012, leading Congress to abandon a proposed anti-piracy bill that was backed by studios and record labels.

Wheeler has faced criticism, even protests, ever since he outlined his proposal late last month. He has scheduled a vote at the FCC’s May 15 meeting to put the proposal into a formal rulemaking process, with the goal of getting new provisions in place by the end of the year.

Whether Wheeler will have the votes to take even that action is still a question. On Wednesday, Commissioner Mignon Clyburn said that the agency has received some 100,000 comments, and in a blog post she pointed out her previous concern that rules prohibit “pay for priority arrangements altogether.” Commissioner Jessica Rosenworcel said in a speech that the proposal has “unleashed a torrent of public response,” and recommended a delay of consideration for at least a month.

“So while I recognize the urgency to move ahead and develop rules with dispatch, I think the greater urgency comes in giving the American public opportunity to speak right now, before we head down this road,” she said.

Ajit Pai and Michael O’Rielly, the two Republicans on the commission, are opposed to net neutrality, saying that the market needs to flourish without regulatory restriction. In an interview with Variety on Wednesday, Pai said that the net neutrality effort were “a solution in search of a problem.”

Update: Pai has called on Wheeler drop the net neutrality proposal from the FCC’s May 15 agenda, saying that he has “grave concerns” about the proposal. Net neutrality still remains on the agenda, but the public is being given more time to comment on it, until the end of the day on May 14.

Outside the FCC, protesters have been camped outside the building, and public interest groups are planning a “day of action” on May 15. Some 100 organizations sent a letter to Wheeler and President Obama, asking them to “reconsider and abandon efforts to adopt rules that would harm, rather than preserve, net neutrality.”

A group of about 50 venture capitalists also sent Wheeler a letter, arguing that “if established companies are able to pay for better access speeds or lower latency, the Internet will no longer be a level playing field.”