Speaking on the podcast The Rest is Politics, Burnham said: "I asked the question coming to Downing Street: well, was there interference in the 2016 referendum from Russia? I've had the clear answer that yes, there was."
Burnham did not disclose who gave him the assessment, what evidence it was based on, or whether any interference had affected the outcome of the referendum. A 2020 report by parliament's intelligence and security committee criticised the government and intelligence agencies for failing to properly investigate potential Russian interference. The cross-party committee found that ministers had failed to seek a full assessment of Moscow's activities despite evidence of attempts to influence British politics through state-backed media and social media accounts.
Burnham said Britain needed to be more open about hostile state interference, particularly as European leaders grapple with the rise of the far right. He discussed the threat from the far-right Alternative für Deutschland (AfD) with German Chancellor Friedrich Merz during a visit to Berlin this week, and said Merz raised concerns about the party's links to Russia. "There is certainly misinformation, disinformation, that is emanating from Russia into German politics," Burnham said.
Last month, the prime minister attacked Russian information warfare during a speech to the UN General Assembly, saying there was evidence Moscow had tried to interfere with the 2019 general election.