Viktor Orbán could face renewed calls for his expulsion from the European People’s party (EPP) at a gathering of the powerful centre-right bloc next month.

The Hungarian prime minister and his Fidesz party will be on the agenda of the EPP political assembly on 20 March, an event intended to approve the group’s manifesto before European parliament elections in May.

Two EPP sources told the Guardian that Orbán and his Fidesz party would be up for discussion, even though no official disciplinary procedure has been launched.

The Hungarian prime minister is facing a growing backlash from centre-right allies after his government launched an aggressive anti-EU poster campaign targeting the European commission leader, Jean-Claude Juncker, who is also in the EPP. And in a one-page letter sent to households in Hungary, Orbán said the EU “has not learned anything from the horrible terror attacks of the past years” and “wants to bring even more migrants to Europe”.

But Orbán’s critics face an uphill battle to expel him from the group. The EPP is Europe’s most powerful political force, with the largest number of seats in the European parliament and nine EU leaders among its ranks.

Centre-right Nordic and Baltic parties are considering disciplinary action against Fidesz, adding their concerns to Dutch and Luxembourgish parties, which have criticised Orbán’s policies in the past.

Petri Sarvamaa, a Finnish Christian Democrat MEP, told the Guardian that Orbán should not be in the EPP. His National Coalition party is considering whether to add its voice to calls for Orbán to go.

“Obviously Fidesz led by Orbán doesn’t share the values of the EPP. I would go as far and call them an undemocratic force.” The practical consequences of keeping Fidesz inside the EPP were “equally important” as the question of values, said the MEP. “What [Fidesz] are trying to do is eating the EPP from the inside. They are like a Trojan horse … it is clear they are in the process of undermining our party group in the European parliament.”

In recent days, German political leaders have also taken a more critical stance, following the launch of the anti-EU poster campaign, which was dismissed by Brussels as “a ludicrous conspiracy theory”.

Angela Merkel’s successor as leader of Germany’s CDU, Annegret Kramp-Karrenbauer told Der Spiegel last week that it was up to Fidesz to prove it still belonged to the EPP. The CDU could break off contact “it’s no longer possible to come to an understanding,” she said. Merkel reproached Orbán by expressing “full solidarity” with Juncker over the posters and said she would “make that clear” in discussions with Hungary.

French EPP president Joseph Daul condemned Orbán last week for “deceitful” and “misleading” claims against the European commission. Daul has previously said it was better to keep an enfant terrible inside the family.

Despite the growing anger, many centre-right parties hold with that view, especially in central Europe.

A source close to the EPP’s parliamentary leader, Manfred Weber, said it was too early to say whether the Bavarian politician would back calls for Fidesz to be kicked out. “There is growing anger in the EPP family,” the source said, adding: “As in the past, [Weber] is very much worried about a split, a further division of Europe between east and west.”

Weber, who aspires to lead the next European commission, issued one of his strongest criticism yet of the Hungarian leader over the poster campaign. “One cannot belong to the EPP and campaign against the current EPP commission president.”

Speaking to journalists last month, the Hungarian government’s chief spokesman, Zoltán Kovács, dismissed suggestions that Fidesz would be forced to leave the EPP. “We belong there, we are members and our election campaign is going to be in line with the European People’s party.”

The procedure to exclude or suspend a political party can be launched by the EPP’s most senior leaders or a joint letter from at least seven parties in five countries.

Anna Maria Corazza Bildt, a senior MEP in Sweden’s Moderate party, told the Guardian that the EPP had spent 10 years trying to bring Orbán into line. “We see that not only is it not improving but the situation in Hungary is becoming more and more disturbing.”

She cited the forced move of the Central European University to Vienna, pressure on independent media and judges, and the “antisemitic tone with the constant demonisation of George Soros”.

“The dismantling of the rule of law is happening, we see the erosion of functional democracy. Democracy is not only winning elections, democracy is the checks and balance, the media. We have tried for 10 years in very good faith [to have a dialogue with the Hungarian government] and we don’t see the result. We have to draw the consequences of that.”