Speaking on his Mediawatch show Thursday night, Mr Neil dismissed the Stop Funding Hate group as opponents of a free society.
He also attacked companies that had given in to their demands to act as the “useful idiots of die-hard censorship fanatics.”
The attack came as the ad boycott showed signs of unraveling, with a series of large companies pulling back from initial claims they had blacklisted the new channel.
Mr Neil, who is the chairman of GB News as well as its main presenter, said Stop Funding Hate started “rounding up the lynching” four months before the channel started broadcasting.
He told viewers, “It’s quite remarkable that serious leaders and established businesses can be so easily intimidated.
“They all got down on their knees to stop funding hate. It is important that they – and you – know who they are in the grip of.
“Stop Funding Hate does not defend a liberal and inclusive society. It is dominated by far-left agitators and cranks who push to boycott advertisers of any media organization with which it does not agree. “
Mr Neil added: “The awakened nonsense has reached the boardroom and corporate capitalism is becoming the useful idiot of die-hard censorship fanatics. “
Moneysupermarket, Vodafone, Bosch and Ikea earlier Thursday sought to distance themselves from the boycott campaign after initially withdrawing their ads. Several companies have said they may choose to resume their media campaigns after a review.
Moneysupermarket, the price comparison site, was the first to reverse its decision to “pause” advertising on GB News.
A spokesperson tweeted: “Just to confirm that Moneysupermarket does not boycott its advertising on GB News, sorry for any confusion caused. “
Swedish furniture retailer Ikea and mobile operator Vodafone have both made it clear that they may relaunch the ads.
A spokesperson for Vodafone said: “Our ad appeared on GB News without our knowledge, and we would have preferred to wait to do a commercial assessment of the channel and decide which ad was the right one, and in what volume. We will do it now.
“We strongly believe in freedom of expression, while firmly opposing hateful and harmful content. We are not involved in any boycott. “
Bosch had previously indicated that he would stop showing his ads “on political channels”.
But in a new statement, the home appliance maker said that even though it ended an ad campaign on GB News earlier, it would now reconsider that position.