MI5 chief Sir Ken McCallum acknowledged Russian interference in the 2016 Brexit referendum but cautioned against inflating its impact on the outcome.
McCallum confirmed that Russian media outlets, trolls and bots attempted to interfere in the vote, describing it as "a matter of public record" and "open source record". However, he warned that portraying such efforts as more substantial than they were risked playing into adversaries' hands.
"There is a really important difference between interference attempted and outcome swayed," McCallum said. He expressed concern that giving Russian trolls continued media attention would be welcomed by Moscow "despite the comparative unimpressiveness" of their actual efforts.
The MI5 chief's comments responded to Prime Minister Andy Burnham's recent assertion that Russian interference in the 2016 referendum was "clear". Burnham told The Rest is Politics podcast that upon taking office he asked whether Russia had interfered and received "the clear answer that, yes, there was". A No 10 spokesperson later clarified that Burnham believed Russian interference attempts were "highly likely" but not successful.
When pressed in the House of Commons, Burnham appeared less definitive about the extent of interference. He denied Conservative accusations that he was questioning the referendum's legitimacy, saying he simply believed Russia had tried to influence the vote. He argued it was inconceivable Russia would have attempted interference in the 2019 general election without trying to meddle in the 2016 referendum.
McCallum agreed strongly that a Russian hack and leak occurred during the 2019 election campaign. He also confirmed that Russian media organisations RT and Sputnik mounted strong operations around the 2016 referendum.
Conservative deputy leader Alex Burghart called McCallum's remarks "an extraordinary slap down to the prime minister". Reform UK leader Nigel Farage accused Burnham of lying, claiming there was "no Russian interference" in the referendum. Former Prime Minister Boris Johnson said the security services told him in 2016 that there was "no evidence of any successful interference" in the referendum.
A 2020 parliamentary report did not conclude firmly on whether Russia attempted to intervene in UK elections and referendums, but criticised the government for failing to assess and counter Russian troll activity on social media.