Twitter has announced it will ban political advertising globally after growing criticism over misinformation from politicians on social media.
On Wednesday, Twitter's chief executive Jack Dorsey said that while internet advertising "is incredibly powerful and very effective for commercial advertisers, that power brings significant risks to politics, where it can be used to influence votes to affect the lives of millions".
Mr Dorsey tweeted that the new policy would ban ads on political issues and those from candidates.
The details are due to be unveiled next month and enforced from November 22, before the UK general election on December 12.
"We considered stopping only candidate ads but issue ads present a way to circumvent," Mr Dorsey said.
"And it isn't fair for everyone but candidates to buy ads for issues they want to push. So we're stopping these too."
Mr Dorsey said the company wanted to avoid problems from "machine learning-based optimisation of messaging and micro-targeting, unchecked misleading information and deep fakes".
Facebook's policy allows political speech and ads to run without fact-checking on the leading social network.
Its chief executive, Mark Zuckerberg, has said political advertising is not a major source of revenue but he believes it is important to allow everyone a "voice", and that banning political ads would favour incumbents.
Mr Dorsey said he disagreed with Mr Zuckerberg's assessment that stopping ads would favour incumbents.
"We have witnessed many social movements reach massive scale without any political advertising. I trust this will only grow," he said.
Mr Zuckerberg shot back quickly, using an earnings conference call Wednesday afternoon to offer an impassioned defense of what he called his company's deep belief "that political speech is important."
"This is complex stuff. Anyone who says the answer is simple hasn't thought about the nuances and downstream challenges," Mr Zuckerberg said. "I don't think anyone can say that we are not doing what we believe or we haven't thought hard about these issues."
Social media platforms have been challenged by US President Donald Trump's use of ads in his campaigns that contain claims critics say have been debunked by independent fact-checkers.
Democrats have stepped up pressure on Facebook to remove the ads, and a group of its employees have also called for stronger efforts to clamp down on "civic misinformation" from politicians.
Some initial reaction to the Twitter announcement was positive.
"Until privately owned social media platforms can develop and consistently enforce standards to prevent demonstrably inaccurate information in political advertising, this is the right move," said Michelle Amazeen, a Boston University professor specialising in political communication.
Nina Jankowicz, a Wilson Centre fellow specialising in use of disinformation, also welcomed the move.
"It's great that this move has been made globally and not just within the United States," Ms Jankowicz said.
"Too often these companies operate in a cloud of wilful ignorance about the effects their products have outside our borders."
She said the decision could level the playing field by preventing wealthier candidates and groups from dominating the conversation.
"Paid speech essentially quashes some groups' ability to speak out and be heard because they can't compete with the reach that their richer counterparts pay for," Ms Jankowicz said.
However, Donald Trump's campaign manager called Twitter's change a "very dumb decision" in a statement.
"This is yet another attempt to silence conservatives, since Twitter knows President Trump has the most sophisticated online program ever," Brad Parscale said.
Shortly after the advertising decision, Twitter announced its tools removed one in two tweets containing abusive content posted in the first half of this year.
Twitter said in its transparency report that it was investing in proactive technology to reduce the burden on people reporting abusive content to the company.
"More than 50 per cent of tweets we remove for abuse are now proactively surfaced using technology, rather than relying on reports to Twitter," the company said. That compared with 20 per cent a year ago.
