Boris Johnson told “so many lies” during the EU referendum campaign, the outgoing president of the European commission, Jean-Claude Juncker, has said.

In a valedictory interview with the German magazine Der Spiegel, Juncker voiced regret he had not countered the claims of the leave campaign in 2016. The commission decided not to get involved on the advice of David Cameron, who feared interventions from Brussels would backfire.

Juncker said that had been a big mistake, adding: “So many lies were told, including by current prime minister, Boris Johnson, that there needed to be a voice to counter them.”

Juncker also pointed the blame for the 2016 result at pro-EU politicians, specifically “my friend” Tony Blair. He said the British, including the former Labour leader, had always seen the EU as an economic project and had shunned political union. “If you stick to that narrative for over 40 years, it should not come as a surprise when people remember it during the referendum.”

The veteran Luxembourg politician, who became commission president in 2014, said he had been sceptical remain would win. Juncker claimed he had told Cameron in 2014: “You are going to lose it.”

Juncker said he had become “an ideological target for Brexiters” and added it was important not to give Europeans “the false impression” that the EU was on the way to becoming a single state: “Even highly enthusiastic Europeans are against our union becoming a European melting pot.”

A wily political operator who has been in politics for four decades, Juncker also revealed how he handled Donald Trump, saying one of his “little tricks” was to use only US trade statistics, so the president could not dispute them.

Juncker also described how he sought to hammer home to Trump that Brussels has sole competence to strike trade deals for EU’s 28 member states. He said Angela Merkel and other EU leaders Trump had spoken to were all important but “the wrong people” to speak to about trade. “That impressed him,” Juncker said.

The commission president said “dumb nationalism” remained a threat to the EU, especially when established parties sought to mimic populists. Juncker, a member of the centre-right European People’s party, did not elaborate on whether he thought his political group had been too soft on colleagues bending the rule of law, such as Hungary’s prime minister, Viktor Orbán.

He sounded a sceptical note about the global school strike movement, whose figurehead is Greta Thunberg, saying: “I really like young people getting involved, but I am also not naive. Much of what is being presented there in such sentimental terms is not so easy to achieve in reality. Classical industry must continue to have a home in Europe.”

In the wide-ranging interview, Juncker also reflected on the trauma of his father, who was forcibly conscripted into the Wehrmacht after Nazi Germany invaded Luxembourg. The shadow of the second world war shaped the outlook of Juncker, who was born in 1954.

He also defended his exuberant greetings, with kisses for autocrats, such as Vladimir Putin and Recep Tayyip Erdoğan. “My staff sometimes warns me not to hug this or that person. But I’ve been kissed by Erdoğan, so I kiss him, too. I kissed Putin and I was kissed by Putin. In either case, it certainly didn’t hurt Europe.”

Asked about persistent rumours that he drank too much, Juncker said “such false claims hurt my family more than me”. Juncker, who has been troubled by poor health during his time in Brussels, is taking a leave of absence for an operation. But his departure as commission president has been delayed, as his successor, Ursula von der Leyen, has encountered problems getting her team approved by the European parliament.