Italy’s prime minister, Giorgia Meloni, has condemned a deal to divide the EU’s top jobs between mainstream pro-European parties, saying it ignored the bloc’s rightward shift.

In an angry speech to the Italian parliament, Meloni said the top jobs deal reflected a view that “citizens are not mature enough to make certain decisions” and had been taken by those who believe “that oligarchy is basically the only acceptable form of democracy”.

Six European leaders, including France’s Emmanuel Macron, Germany’s Olaf Scholz and Poland’s Donald Tusk, on Tuesday confirmed a plan to nominate the centre-right politician Ursula von der Leyen for a second term as European Commission president and split two other key posts between the Socialists and liberals.

Meeting before an EU summit on Thursday, the six leaders, who also included the prime ministers of Spain, the Netherlands and Greece, were lead negotiators representing the centre-right European People’s party, the Socialists and liberals, who together hold 55% of the seats in the new European parliament.

Meloni is the president of the European Conservatives and Reformists, which has become the parliament’s third largest group, bigger than Macron’s liberal Renew group. But the Eurosceptic ECR was left out of the top jobs deal.

“The third [largest] group today is a group that is not liked by those who are deciding,” Meloni said, calling the EU a “bureaucratic giant” whose choices were dictated by ideology.

She highlighted that her governing coalition, led by the Brothers of Italy party, took almost 53% of the European vote in Italy, whereas the governing parties of France took only 16% and Germany 32%.

Despite Meloni’s frustration, her government has never put forward an obvious candidate for any of the three jobs at stake this week.

Diplomatic sources said Meloni had already agreed with the Czech Republic’s prime minister, Petr Fiala – the only other ECR leader around the EU summit table – to support the top jobs package, as long as both countries got powerful jobs in the next European Commission.

Sources also suggested that the ECR had some leverage over von der Leyen, who cannot take up a second term without confirmation from the European parliament.

Von der Leyen may seek Meloni’s support in the European parliament vote on her appointment, which is expected next month. While the pro-European groups that back her hold a majority of seats, analysts say von der Leyen must broaden her base, as it is likely that she will lose votes from disgruntled MEPs belonging to the groups supporting her.

Von der Leyen could also seek votes from the Greens, who have said they will not support her if she works with the ECR.

Meloni’s criticism echoes that of Hungary’s prime minister, Viktor Orbán, who has claimed the deal between the mainstream party “sow[ed] the seeds of division”. Like Meloni, Orbán’s MEPs sit outside the pro-European groups, but the two Eurosceptic leaders have been unable to agree to join forces, blunting their opposition.

An EU diplomat stressed that the three political groups that came up with the deal – the EPP, Socialists and liberal Renew – represent 22 of the 27 heads of state and government and 80% of the bloc’s population.

“I do hope it’s stable,” the diplomat said, who also cautioned that it would be a mistake for this week’s summit to get into haggling over portfolios in the new commission.

“I think it would be wrong to enter during this summit into the discussion on the composition of the college of commissioners,” the person said.

Speaking in the run-up to parliamentary elections in France, the diplomat pointed to “anxiety” and “fears” about forthcoming votes in some EU members states and warned about avoiding delays.

“This is indeed the moment when the issue of top jobs should not really take too much of our attention. The decision should be taken tomorrow,” the diplomat said, adding: “To have another round … makes no sense. I believe that the stakes are higher than that.”

Despite Meloni’s irritation and Orbán’s outright opposition, the majority of EU leaders meeting on Thursday are expected to confirm the jobs deal, which would also make Estonia’s Kaja Kallas the bloc’s chief diplomat and Portugal’s António Costa president of the European Council.

Orbán has already said he will not support von der Leyen; Meloni has not made her position clear, but the Italian newspaper La Stampa reported it was likely she would abstain. Sources in Brussels were not able to immediately confirm or deny this report.

Previous European Commission appointments have gone forward despite opposition, even from large member states. In 2014, the then Conservative prime minister, David Cameron, joined Orbán in an unsuccessful attempt to stop Jean-Claude Juncker from becoming head of the European Commission.