Vodafone (VOD) has set out new global rules to prevent its advertising from appearing alongside extremist content, as the U.K. prime minister takes aim at tech firms following Saturday evening's terrorist attack in London.

Vodafone is one of the world's largest telecommunications companies and is among the first to publicly react with a change in policy.

The U.K.-based company's new rules include a definition of hate speech and "fake news" for the purpose of determining whether an outlet should carry Vodafone advertising.

Vodafone Tuesday said it was taking a whitelist-based approach to content control, providing its global agency network, led by WPP (WPPGY) , Google (GOOGL - Get Report) and Facebook (FB - Get Report) a list of outlets on which it will would like its ads featured.

"Those controls ensure that Vodafone advertisements are only served within selected outlets identified as highly unlikely to be focused on harmful content," Vodafone said. "These measures will be reviewed regularly by Vodafone and its global agency network to ensure that the selection of outlets for whitelisting is appropriate and neither too broad nor too narrow."

The company said that third parties and advertising platform suppliers, including Google and Facebook, should take all measures necessary to ensure Vodafone's advertising does not appear within hate speech and fake news outlets.

Vodafone defined these outlets as "deliberately intended to degrade women or vulnerable minorities ("hate speech"); or presented as fact-based news (as opposed to satire or opinion) that has no credible primary source (or relies on fraudulent attribution to a primary source) with what a reasonable person would conclude is the deliberate intention to mislead ("fake news")."

Vodafone shares were 0.41% up at 11:11 BST in London, extending a three-month gain of 14.48%.

The move comes two days after U.K. Prime Minister Theresa May put big technologies companies in the cross hairs of the fight on terrorism, saying they have to do more to prevent hate speech online.

"We cannot allow this ideology the safe space it needs to breed," May said in a Sunday statement outside Number 10 Downing Street in the wake of Saturday's terror attack in the London Bridge and Borough Market area of the capital.

"Yet that is what the internet - and the big companies that provide internet-based services - provide," May added. "We need to work with allied, democratic governments to reach international agreements that regulate cyberspace to prevent the spread of extremism and terrorist planning.

"And we need to do everything we can at home to reduce the risks of extremism online," she added.

Facebook and Alphabet Inc.'s (GOOGL - Get Report) YouTube have come under fire in recent months for not doing enough to prevent extremist activity and remove content.

In a reply to May's statement, which was posted on Facebook, the company's policy director wrote: "Using a combination of technology and human review, we work aggressively to remove terrorist content from our platform as soon as we become aware of it.

"Online extremism can only be tackled with strong partnerships. We have long collaborated with policymakers, civil society, and others in the tech industry, and we are committed to continuing this important work together."

In early May, after a row erupted with advertisers after commercials were shown alongside extremist content, Facebook hired 3,000 new people to moderate and remove inappropriate content.

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