Donald Trump’s former chief strategist Steve Bannon advised Italy’s interior minister Matteo Salvini to attack the pope over the issue of migration, according to sources close to the Italian far right.

During a meeting in Washington in April 2016, Bannon – who would within a few months take up his role as head of Trump’s presidential campaign – suggested the leader of Italy’s anti-immigration League party should start openly targeting Pope Francis, who has made the plight of refugees a cornerstone of his papacy.

“Bannon advised Salvini himself that the actual pope is a sort of enemy. He suggested for sure to attack, frontally,” said a senior League insider with knowledge of the meeting in an interview with the website SourceMaterial.

After the meeting, Salvini became more outspoken against the pope, claiming that conservatives in the Vatican were on his side. One tweet from Salvini’s account, in May 2016, said: “The pope says migrants are not a danger. Whatever!” On 6 May 2016, Salvini, after the pope’s plea for compassion towards migrants, stated: “Uncontrolled immigration, an organised and financed invasion, brings chaos and problems, not peace.”

Facebook Twitter Pinterest Matteo Salvini, right, with the controversial T-shirt. Photograph: @matteosalvinimi

The claims coincide with suggestions that Bannon’s pan-European populist project, the Movement, has stalled. Meanwhile, Salvini has announced that he wants to bring the far right from across Europe into an alliance. Last Monday in Milan, he unveiled his “vision of Europe for the next 50 years”, billing it as the launch of a new rightwing coalition for the European parliamentary elections on 23 May. Salvini unveiled his alliance only days after meeting Bannon in Rome in March. This led some to believe that Bannon has handpicked Salvini as the informal leader of Eurosceptic, populist forces in Europe.

The pair also met in Rome six months ago, prompting Mischaël Modrikamen, the Movement’s managing director, to tweet that Italy’s deputy prime minister “is in!”

Bannon, in an interview with NBC and SourceMaterial to be broadcast at 9pm eastern time in the US on Sundayon , also takes issue with the pope’s warnings over resurgent populist movements. “You can go around Europe and it’s [populism] catching fire and the pope is just dead wrong,” said Bannon.

Following the September 2016 meeting between Salvini and Bannon, the League leader was photographed holding up a T-shirt emblazoned with the words: “Benedict is my pope.”

The slogan refers to a Vatican version of the “birther” campaign waged by Trump against Barack Obama, claiming that Francis’s papacy is illegitimate and that his ultra-conservative predecessor Benedict XVI is in fact the true pontiff.

The League source also alleged that Salvini would have attacked the pope harder but was restrained by his own party, predominantly by Giancarlo Giorgetti, the deputy federal secretary of Lega Nord who is close to senior figures in the Vatican.

Facebook Twitter Pinterest Pope Francis greets women in Morocco, as part of a trip aimed at showing solidarity with migrants at Europe’s door. Photograph: Gregorio Borgia/AP

“[After the Bannon meeting] Salvini moved very tough and said: ‘We have to attack the Vatican, but the other guy said wait.’ Salvini thinks by himself and acts by himself ... so he started to act [for example, by appearing with the ‘Benedict is my Pope’ T-shirt],” said the source.

Bannon has steadily been building opposition to Francis through his Dignitatis Humanae Institute, based in a 13th-century mountaintop monastery not far from Rome.

In January 2017, Bannon became a patron of the institute, whose honorary president is Cardinal Raymond Burke, an ultra-conservative who believes organised networks of homosexuals are spreading a “gay agenda” in the Vatican.

The institute’s chairman is former Italian MP Luca Volontè, on trial for corruption for accepting bribes from Azerbaijan . He has denied all charges.

Among the institute’s trustees is one of the pope’s most outspoken critics, Austin Ruse, a former contributor to rightwing news website Breitbart. Ruse runs C-FAM, an anti-abortion group whose founder was prone to antisemitic rants about population control and which has been termed a hate group by human rights campaigners. Like Volontè, Ruse is an official of the World Congress of Families, a gathering of far-right, anti-gay Christian groups backed by Konstantin Malofeev, a Russian oligarch close to Vladimir Putin.

Other trustees include Ben Harris-Quinney, chairman of the Conservative thinktank the Bow Group, who met Bannon in London last summer, alongside Raheem Kassam, the former UK editor of Breitbart. Bannon was a founding member of Breitbart’s board.

Bannon was invited by the Observer to respond but at the time of publication had not yet replied.