Boris Johnson told “so many lies” during the Brexit referendum campaign, says Jean-Claude Juncker.
The outgoing president of the European Commission said he regretted not getting involved in the campaign ahead of the historic vote in June 2016.
It is the second time in two weeks Mr Juncker has accused the prime minister of spreading lies before the referendum.
Mr Juncker said he had been “firmly convinced” that the referendum “would go wrong”.
His term at the helm of the European executive will come to an end by January 2020.
The former prime minister of Luxembourg also revealed he made £1 by betting on Britons to vote to leave the European Union three years ago.
Mr Juncker told German magazine Der Speigel: “I was one of those early on who was firmly convinced that this referendum would go wrong.
“When then-prime minister David Cameron told me on the sidelines of the 2014 G20 summit in Brisbane that he really wanted to hold a Brexit referendum, I told him: ‘You’re going to lose it.’
“I made a bet with the European commissioner of British nationality at the time, Jonathan Hill: I get a pound from you if the Remainers lose, you get a euro if you win.
“I have that pound today.”
Asked why he opted not to get involved in the Remain fight, Mr Juncker said he had “many invitations” to campaign.
He said: “Cameron made it clear that he didn’t have any use for me.
“The European Commission is even less popular in Britain than it is on the continent. I decided not to get involved.
“Looking at it today, I think that was a big mistake. So many lies were told, including by current prime minister Boris Johnson, that there needed to be a voice to counter them.”
Britain’s departure from the European Union was initially due to take place on March 29 this year, but was later postponed to May 22, and then October 31.
A so-called flextension has been granted until January 31, and a general election is behind held on December 12.