Donald Tusk has given his implicit backing to Boris Johnson’s opponents in the general election with a call for anti-Brexit campaigners to keep fighting in the month before Britain goes to the polls.

In what he openly conceded was an unconventional move, the outgoing president of the European council made a pointed intervention in the UK’s general election debate with a thinly veiled message of solidarity for those seeking to unseat Johnson’s Conservatives.

Reflecting on his five years in his role as a top EU official as it comes to an end this month, and following a speech on the life of the journalist and philosopher Hannah Arendt, Tusk said he felt empowered to be honest about his feelings.

He added that he had been jealous of the renewed freedom of expression of John Bercow since standing down as Commons Speaker.

“I want to tell you something I wouldn’t have dared to say a few months ago, as I could be fired for being too frank,” Tusk said. “The UK election takes place in one month. Can things still be turned around? Hannah Arendt taught that things become irreversible only when people start to think so. So the only words that come to my mind today are simply: don’t give up. In this match, we have already had added time, now we are in extra time, perhaps it will even go to penalties?”

Tusk’s comments mark a break from the policy of EU chiefs steering clear of getting drawn into domestic political debates in member states during election periods.

But the former Polish prime minister, who also cited Margaret Thatcher as among his heroines during his speech, has long been an outspoken critic of those who he claims misled the British public into backing Brexit and turning their backs on the EU.

Speaking at the College of Europe, a training ground for a string of European leaders and senior officials, Tusk said he heard all around the world, and specifically in those countries that were once part of the British empire, that Brexit would leave the UK as an “outsider, a second-rate player”.

“I have heard repeatedly from Brexiters that they wanted to leave the European Union to make the United Kingdom great again, believing that only alone, it can truly be great,” Tusk said. “You could hear in these voices a longing for the empire. But the reality is exactly the opposite.”

He went on: “Only as part of a united Europe can the UK play a global role, only together can we confront, without any complexes, the greatest powers of this world.

“In fact, I can say the same about Germany or France. And the world knows it. I have heard the same in India, New Zealand, Australia, Canada and South Africa; that after its departure, the UK will become an outsider, a second-rate player, while the main battlefield will be occupied by China, the US and the EU.

“‘Why are they doing this?”’ – I was asked this regretful question everywhere I went. One of my English friends is probably right when he says with melancholy that Brexit is the real end of the British empire.”

Tusk also defended his hardline approach to Vladimir Putin during his tenure in Brussels as convenor of EU summits, and organiser of the bloc’s positions on key affairs.

The Russian leader’s strategic goal, Tusk said, was “not only to regain control of the former Soviet Union territories, but also to systematically weaken the EU by provoking internal divisions”.

“I had to publicly remind others, almost every week, that Russia is not our ‘strategic partner’, but our ‘strategic problem’,” Tusk said. “I was even labelled a ‘monomaniac’, for being so focused on this subject. But eventually it paid off.”

Tusk, a one-time member of Solidarity, was briefly imprisoned by the Communist government in Poland in the 1980s. Russia invaded Ukraine around the time Tusk took office in Brussels. Among those who have offered a softer line on how to deal with the Kremlin has been Jean-Claude Juncker, the outgoing president of the European commission.

Tusk was scathing about the recent comments of the French president, Emmanuel Macron, who had suggested a rethink of the relationship with Russia.

“When I hear Macron’s words, that ‘we must reconsider our position with Russia, to rethink the strategic relationship’, I can only express hope that it will not happen at the cost of our common dreams about Europe’s sovereignty,” Tusk said.

“In the same interview for the Economist, President Macron says that he shares the same views on this subject as Viktor Orbán, and that he hopes that Mr Orbán will help convince Poles to change their position on Russia. But not me, Emmanuel.”