More than 10m tweets sent by state actors attempting to influence US politics have been released to the public, forming one of the largest archives of political misinformation ever collated.

The database reveals the astonishing extent of two misinformation campaigns, which spent more than five years sowing discord in the US and had spillover effects in other national campaigns, including Britain’s EU referendum.

Twitter announced on Wednesday that it was making the tweets available to researchers and the public to support broader analysis of how misinformation campaigns operate.

The archives contain tweets sent between 2013 and 2018 by 3,800 accounts associated with the St Petersburg-based Internet Research Agency, which generated 9m tweets; and 700 accountsassociated with a smaller misinformation campaign backed by Iran, which produced 1m tweets. Including the videos and images the political trolls uploaded, the database comes to more than 350GB in size.

The focus of both campaigns was on the US, but their “opportunistic” nature meant that others found themselves in the firing line, according to the Washington DC-based Digital Forensic Research Lab (DFRLab).

“The Russian trolls often chose targets of opportunity, especially elections and terrorist attacks, in their attempts to interfere in local politics,” the lab said after analysing the database. “This included promoting anti-Islam hashtags after the Brussels terror attacks, a pro-leave hashtag on the day of Britain’s Brexit referendum, and leaks targeting the French president, Emmanuel Macron, before his election.

“On June 23, 2016, as Britain held its Brexit referendum, the troll farm’s accounts posted #ReasonsToLeaveEU 1,102 times, a mixture of authored tweets and retweets. They were apparently spearheaded by @WorldOfHashtags, which posted, ‘Everybody is obsessed with #EUref today. So let’s play #\ReasonsToLeaveEU.’

“This appears to have been an attempt, on voting day, to make a pro-leave hashtag trend. However, it should not be taken as a larger Russian attempt to interfere in Brexit. The Russian troll farm only posted on #VoteLeave 35 times in its career and ‘Brexit’ 4,437 times and mostly after the vote, suggesting that there was no concerted campaign around the issue.”

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The DFRLab said the archive underscored the difficulty in finding and fighting such an operation in real-time. “The most effective Russian trolls used exactly the techniques which drive genuine online activism and engagement. That made it much harder to separate them out from genuine users. It will continue to do so. Identifying future foreign influence operations, and reducing their impact, will demand awareness and resilience from the activist communities targeted, not just the platforms and the open source community.”

The trolling was not confined to social media. As a previous Guardian analysis has shown, Russian trolls were cited in more than 100 UK news articles by unknowing authors, who incorporated their tweets as examples of humour, “on-the-ground” reporting, and public opinion.

One account, @KaniJJackson, posed as a Black Lives Matter activist and tweeted under the names “Kanisha J” and “Remove Trump Now”, with a profile picture of Michelle Obama. That was cited nine times by publications including BuzzFeed, the Daily Mail and the Daily Express. Another, @SouthLoneStar, pretended to be a US white supremacist and was cited by the Metro, the Telegraph and the Sun over provocative tweets sent in the wake of the Westminster Bridge terrorist attack.