MI5 chief Sir Ken McCallum has said there was Russian interference in the the Brexit referendum, but warned against overstating its impact on the result.
It comes after Prime Minister Andy Burnham provoked a backlash from some Brexit campaigners by saying it was “clear” there had been interference by Moscow in the 2016 vote.
Sir Ken said it was “a matter of public record” that there was interference in the referendum by online trolls and bots.
However, he suggested Russian trolls would be “delighted” to be front page news again, “despite the comparative unimpressiveness” of their efforts.
He added: “There is a really important difference between interference attempted and outcome swayed.
“I am a little bit anxious that we don’t play into our adversaries’ hands by making these trolls look 10ft tall.”
Conservative deputy leader Alex Burghart branded Sir Ken’s comments “an extraordinary slap down to the prime minister”.
In an interview released on Monday, Burnham told The Rest is Politics podcast, external that upon arriving in Downing Street, he had asked if there had been Russian interference in the 2016 vote, adding: “I’ve had the clear answer that, yes, there was.”
He did not provide details about the extent or nature of Russian activities during the interview, and when pressed on the subject in the House of Commons on Monday afternoon appeared to be less definitive.
The PM denied claims from Conservative leader Kemi Badenoch that he was implying the referendum result was illegitimate because of Russian interference.
He said it was a “matter of record” that Russia sought to interfere in the 2019 general election, adding: “We think it is inconceivable that this was the first time”.
A No 10 spokesperson also clarified that while the prime minister believed it was highly likely Russia tried to influence the referendum result, he did not believe those efforts were successful.
The Conservatives accused Burnham of making an “embarrassing retreat” after “being caught out trying question the legitimacy of the referendum result”.