Brad Parscale, Mr. Trump’s campaign manager, lashed out on Twitter at Google for the policy change, accusing political elites and big tech of wanting to “rig the election” and said they “won’t stop until they control all digital political speech.” Earlier in the day, the Trump campaign also criticized Facebook in a tweet for considering changes to its political advertising policy, saying it “wants to take important tools away from us for 2020.”

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Tom Channick, a Facebook spokesman, said the company was “looking at different ways we might refine our approach to political ads.”

Google said while it had never allowed any advertiser — including politicians — to make false assertions, it was clarifying its policy to explicitly ban ads that make “demonstrably false claims that could significantly undermine participation or trust” in the election process. Google noted, however, that it expects to take action on a “very limited” number of political ads. Michael Posner, a professor at New York University’s Stern School of Business, said Google’s policy change was a good start but did not go far enough in dealing with potential misinformation.

“It feels too much like a lawyer looking for language to give the company a lot of latitude,” Mr. Posner said.

Google said it would introduce the new policies within a week in Britain, ahead of the country’s general election, and then to the United States and the rest of the world in January, shortly before the Iowa caucuses. The company said the ad policy would include any candidates or officeholders of state or federal elected office.

In an email to campaigns, Google outlined the new rules in greater detail, including that election ads will no longer be allowed to target what is called “affinity audiences” that look like other groups that campaigns might want to target. Campaigns can also no longer upload their own lists of people to show ads.

In addition, there will be no more of what is known as “remarketing,” the process of serving ads to people who have previously taken an action like visiting a campaign’s website.