LONDON — More than 150,000 Russian-language Twitter accounts posted tens of thousands of messages in English urging Britain to leave the European Union in the days before last year’s referendum on the issue, a team of researchers disclosed on Wednesday.
More than 400 of the accounts that Twitter has already identified to Congressional investigators as tools of the Kremlin, other researchers said, also posted divisive messages about Britain’s decision on withdrawing from the bloc, or Brexit, both before and after the vote.
Most of the messages sought to inflame fears about Muslims and immigrants to help drive the vote, suggesting parallels to the strategy that Russian propagandists employed in the United States in the 2016 election to try to intensify the polarization of the electorate.
The separate findings amount to the strongest evidence yet of a Russian attempt to use social media to manipulate British politics in the same way the Kremlin has done in the United States, France and elsewhere.
The disclosures came just two days after Prime Minister Theresa May of Britain delivered a speech accusing Russia of using cyberattacks and online propaganda to “undermine free societies” and “sow discord in the West.”
On Tuesday, the chief of the National Cyber Security Center released a summary of a prepared speech asserting that in the past 12 months, Russian hackers had unleashed cyberattacks on the British energy grid and the telecommunications and media industries.
Taken together, the flurry of reports and accusations adds to growing pressure on Twitter, Facebook and other social media companies to disclose more of their internal records about advertising payments and account registrations, information essential to illuminating the extent of Russian meddling in the referendum on Brexit.
Any evidence that Moscow did, however, may also complicate the already vexed politics surrounding the issue.
Social media companies disclosed Russia’s role in the American election only after prodding by Congress. Facebook, for example, disclosed this year that, over 32 months, the Kremlin-linked Internet Research Agency had paid more than $100,000 for online advertising, posted 80,000 pieces of divisive content and reached more than 126 million Americans.
The Kremlin has long sought to weaken or divide the European Union, perceiving it as a rival for influence in countries from the former Soviet bloc. And Russia’s main propaganda outlets like Sputnik and Russia Today pushed hard for Brexit.
But the British government, consumed by the negotiations for an exit from the European Union, has not yet obtained similar disclosures. Although a parliamentary committee recently asked the social media companies for information, many critics have argued that the government has little appetite for an inquiry that could muddy its mandate.
The social media companies, including Facebook, Twitter and Google, have had little incentive to volunteer information about the exploitation of their own platforms. And the predominantly right-wing, pro-Brexit British press, particularly the powerful tabloids, have little enthusiasm for undermining the validity of the referendum.
That dynamic may now be changing. The Times of London reported on Wednesday that a team of researchers working on an unrelated and still-unpublished study had identified 156,252 Twitter accounts that listed Russian as their language but posted messages in English to argue against the European Union.
The researchers, Oleksandr Talavera and Tho Pham, of Swansea University in Wales, said in interviews on Wednesday that the Russian accounts, which had posted very little about Brexit in the month before the referendum, became quite active at the last minute: from about 1,000 a day two weeks before the vote to 45,000 in the last 48 hours and 39,000 on the day the results were announced, June 24, 2016.
“It is very strange that someone whose language is Russian tweets in English,” Ms. Pham said — describing it as an anomaly that had prompted the researchers to look more closely at 10 of the most active accounts under suspicion.
Nine had been deleted, and the last, Sveta1972, appeared to emanate from a Russian resort town. On the eve of the referendum, Sveta1972 urged Britain to “make June the 23rd our Independence Day.”
Related findings by a separate team of researchers were reported this week by The Guardian, a strong opponent of Brexit. Those researchers had collected a sample of more than 60 million Twitter posts using hash tags related to Brexit.
They found that 419 of the 2,752 accounts already identified by Twitter to Congressional committees as being linked to the Kremlin had posted 3,500 times using Brexit hash tags.
More than 70 percent of those posts were after the vote, but 38 of the accounts tweeted a total of 400 times on the day of the vote, said Laura Cram, a professor at the University of Edinburgh, who conducted the study with her colleague, Clare Llewellyn.
Researchers study Twitter for indications of broader patterns in social media, because information about its users and contents is more accessible than in the larger but less-open platforms like Facebook. But without the disclosure of more information from the companies, social scientists said, neither of the findings disclosed this week will provide conclusive evidence of a deliberate Kremlin campaign to influence the Brexit vote or other British elections.
Ms. Cram, for example, noted that Russian propagandists might have used hash tags related to Brexit only because they fit with protectionist, nativist or anti-Muslim themes that resonated in the context of the American election, as well.
“These accounts were definitely using Brexit hash tags,” she said, “But we cannot say whether they were primarily trying to influence Brexit or whether it was a side effect of them trying to wreak discord generally.”
President Vladimir V. Putin of Russia has denied trying to influence the Brexit vote. But concerns about Russia’s use of social media to meddle in elections have become so commonplace that the Spanish government recently accused the Kremlin of playing a significant role in the push for Catalan independence.
A third, smaller survey, reported last week by Wired Magazine, found that 29 of the Russian-linked Twitter accounts identified to Congress had also tweeted 139 times about Britain or Europe. Blaming terrorist attacks on Islam or railing against immigrants were favorite themes, said Jonathon Morgan, chief executive of New Knowledge, a digital security firm based in Austin, Tx., that conducted the survey.
“Many of these accounts strongly pushed the narrative that all Muslims should be equated with terrorists and made the case that Muslims should be banned from Europe,” he said. “That is very consistent with the Russian strategy on U.S. issues, and the fact that they were using the same tactics to target Europeans is extremely telling.”