A growing avalanche of advertisers are boycotting Google for placing ads next to extremist and otherwise offensive content — and it could turn into a billion-dollar problem, according to one Wall Street analyst.

AT&T, Verizon, pharma giant GSK and the Enterprise car rental company all said Wednesday they were pulling their ads from a variety of Google platforms including the YouTube video-sharing site until they are satisfied that their brands aren’t popping up next to internet garbage.

A Times of London report Wednesday showed that Verizon ads were displayed next to a radical Egyptian cleric who is banned from the US. Google had disabled UK advertisers, but US advertisers continued to show up there.

AT&T’s ads, meanwhile, were found on a website featuring al Qaeda operatives.

This week, Brian Wieser of Pivotal Research was the first analyst to downgrade Google because of the dustup.

It began last week when marketing giant Havas pulled all of its ads from Google’s YouTube UK site following a report from the Times of London that British government ads had been placed alongside videos from white supremacist David Duke and anti-Semitic pastor Steve Anderson.

Wieser told The Post on Wednesday that the impact will be felt by Google, which instead of discussing how to take away TV ad dollars in upcoming ad negotiations will now be mired in “brand safety issues.”

“The marginal increase [in ad dollars] is less likely to occur,” Wieser said.

He said the impact on Google was likely to be around 1 percent of its revenue base, or $1 billion.

“Google’s stated solution was late and woeful,” Wieser said. “Given numerous opportunities to nip it in the bud, they’ve seemingly made it worse.”

The two telecom giants — which compete with Google for ad dollars — issued statements Wednesday.

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

AT&T, which said it is pulling all of its ad spending from Google except for search ads, is the nation’s second-largest advertiser behind Procter & Gamble, spending some $3.3 billion on paid advertising, according to Advertising Age. Verizon ranked No. 5, spending $2.5 billion.

“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,” Verizon said in a Wednesday statement.

“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.”

A spokeswoman declined to say where Verizon’s ads were appearing. Verizon just acquired Yahoo and AOL in order to compete with Google for ad dollars.

Google has apologized for the problem and has said it will revamp tools for advertisers to help correct the issues.

“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,” a Google spokesperson said Wednesday.