The boycott of Google's parent company, Alphabet, over its video service YouTube came into sharp focus in the aftermath of the London terrorist attack with a warning Friday from British Prime Minister Theresa May.

More than 260 organizations worldwide, including the British government, McDonald's and Verizon, already had pulled advertising from YouTube because Google may have placed their brands alongside videos promoting terrorism and other offensive content.

Now, May is warning Google and other Internet giants that they "must do more" to stop extremist material from being posted online.

Shares of GOOGL have fallen nearly 4 percent since March 17, when advertisers began pulling their market campaigns from YouTube and Google. Wall Street's Pivotal Research Group, meanwhile, has downgraded Alphabet shares from "buy" to "hold" based on its response to the boycott.

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Google is the Internet's largest seller of advertising, generating $22.4 billion in ad sales in the fourth quarter of 2016, about 85 percent of the total revenue in the period by parent company Alphabet.

The Daily Mail of London reported Thursday how easy it was after the London attack to find guides via Google and Twitter for carrying out a car terror attack.

The manuals urge jihadists to deploy large vehicles as "tools of war" before going on a stabbing rampage, which was precisely what 52-year-old Muslim convert Khalid Masood did Wednesday.

Masood mounted the sidewalk on the Westminster Bridge, killing three people and wounding 20 others before crashing his rented four-wheel drive vehicle into a gate at the Parliament building and stabbing a police officer to death. Masood was shot and killed by police on the scene.

Prior to the attack, the Daily Mail noted, Google had been forced to promise it would take a "tougher stance" on "extremist" material in response to the boycott.

The boycott was prompted by an investigation by the Times newspaper of London that found advertisements of major brands alongside videos by ISIS preachers and white supremacists.

The Times investigators said it flagged six "virulently antisemitic" videos on YouTube last week, but Google ignored the notifications. Two of the videos were left in place even after the Times contacted Google.

Google, the Times said, not only was paid for the ads, it channeled funds directly into the hands of the extremists.

The prime minister's official spokesman said the "fight against terrorism and hate speech has to be a joint one."

"The government and security services are doing everything they can. It's clear that social media companies can and must do more," he said. "Social media companies have a responsibility when it comes to making sure this material is not disseminated.

"The ball is now in their court. Let's see how they respond," he said.

Foreign Secretary Boris Johnson said Thursday Internet providers and social media companies have "got to look at the stuff that's going up on their sites, they've got to take steps to invigilate it, to take it down where they can."

Americans follow European exit

Joining several European advertisers, AT&T and Johnson & Johnson were among the biggest U.S. advertisers to announce Wednesday that they would stop their ads, with the exception of search ads, from running on YouTube and other Google properties.

The day before, the New York Times reported, Google outlined steps it would take to stop ads from running next to “hateful, offensive and derogatory content.”

Google promised to improve, but the advertisers wanted to be assured that there was zero risk that their ads would appear next to extremist content, Brian Wieser, a media industry analyst at Pivotal Research, told the New York Times.

"They're saying they're trying harder — that's insufficient," Wieser said of Google. "They don't seem to understand the scale of the perceived problem."

Google has insisted it prevents ads, which are placed on websites automatically, from running near inappropriate material "in the vast majority of cases."

It also tried to provide some context, pointing out that it adds thousands of sites to its advertising network every day along with about 400 hours of video to YouTube every minute.

Nevertheless, the British government, the Guardian, the pharmaceutical company GSK and the French advertising multinational Havas are among the European advertisers that have pulled spending.

AT&T said in a statement: “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 nonsearch platforms.”

Johnson & Johnson said it “has decided to pause all YouTube digital advertising globally to ensure our product advertising does not appear on channels that promote offensive content."

"We take this matter very seriously and will continue to take every measure to ensure our brand advertising is consistent with our brand values," the company said.

Google responded, promising a review of its policies.

"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."

Obama partnership

In contrast to European nations, Google created a "remarkable partnership" with the Obama White House, the Intercept reported last April, providing expertise, services, advice and personnel for vital government projects.

A strong case to sanction Google was quashed by a presidentially appointed commission, the Intercept said, while in the European Union, Google was facing two major antitrust charges for abusing its dominance in mobile operating systems and search.

Teaming up with Campaign for Accountability, the Intercept found Google representatives attended White House meetings more than once a week, on average, from the beginning of Obama’s presidency through October 2015.

Nearly 250 people had moved from government service to Google employment or vice versa over the course of the Obama administration, the Intercept said.

"No other public company approaches this degree of intimacy with government."

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