AT&T and Verizon are the latest companies to pull advertising from Google’s display network amid concerns that company does not do enough to prevent ads from appearing on YouTube videos promoting terrorism and hate speech.

The two telecommunication conglomerates issued statements on Wednesday that they will stop advertising on YouTube and Google’s display network until they are reassured that their ads won’t play ahead of videos with objectionable content.

The problem was highlighted last week when The Times published an article demonstrating how often Google’s programmatic advertising tools fail to prevent ads from appearing in front of videos by terrorists and hate groups.

In response, Google promised to give brands more control over where their advertisements run and make it easier for them to alert the company when ads appear in problematic places.

A Google spokesperson told TechCrunch “We don’t comment on individual customers but as announced, we’ve begun an extensive review of our advertising policies and have made a public commitment to put in place changes that give brands more control over where their ads appear. We’re also raising the bar for our ads policies to further safeguard our advertisers’ brands.”

As AT&T and Verizon’s decision to join the boycott demonstrates, however, advertisers aren’t satisfied yet with Google’s efforts–and that has the potential to hurt the company’s ad revenue. According to Kantar Media data cited by Bloomberg, Verizon and AT&T are the third- and fourth-largest advertisers in the United States, respectively, with AT&T alone spending $941.96 million on advertising last year.

The diverse group of companies and organizations already boycotting Google ads include the U.K. government, advertising and marketing agency Havas SA, Sainbury’s, Toyota Motor, Volkswagen, GlaxoSmithKline, the British Broadcasting Corporation and the Guardian newspaper.

In a media statement, AT&T said “We are deeply concerned that our ads may have appeared alongside YouTube content promoting terrorism and hate. Until Google can ensure that this won’t happen again, we are removing our ads from Google’s non-search platforms.”

In its own release, Verizon stated “We take careful measure to ensure our brand is not impacted negatively. Once we were notified that our ads were appearing on non-sanctioned websites, we took immediate action to suspend this type of ad placement and launched an investigation. We are working with all of our digital advertising partners to understand the weak links so we can prevent this from happening in the future.”

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