Ken McCallum, the director general of Britain's domestic intelligence agency, made the remarks during an annual threat update on Tuesday. He endorsed recent comments by Prime Minister Andy Burnham, who had raised the issue after receiving what he described as a clear answer from Downing Street that Russia did interfere in the vote.
McCallum said the Russian-owned English-language broadcasters RT and Sputnik "came in pretty strongly around the time of the referendum" with biased anti-migration and anti-EU reports. He also noted a second line of interference through internet trolls operating largely on X, though the number of fake accounts was unspecified.
"Neither tactic was particularly sophisticated or successful," McCallum said, emphasising a big difference between "what may have been attempted and what actually resulted".
The confirmation follows years of studies and reports into Russian interference. The digital campaign group 89up previously said RT and Sputnik published 261 mostly misleading articles that achieved 134 million potential page impressions. Other studies have reported on a state-linked Russian "troll army" tweeting about the referendum, though estimates of the number of accounts active have varied wildly.
Burnham first raised the issue on Sunday in a podcast interview, but later backed off when challenged by the Conservatives over whether this meant he regarded the Brexit vote as illegitimate. On Tuesday, Downing Street said it was confirmed Russia interfered in the 2019 general election and it was "inconceivable that they didn't in 2016", but added: "We don't believe it was successful in the British referendum in 2016."
McCallum cautioned that continued debate over the issue gave the Kremlin a needless propaganda victory. "It is up to us how far we allow our adversaries to divide us," he said.