A small far-right party that unexpectedly entered the Greek Parliament last year will not be allowed to field candidates for the European Parliament this summer after Greece’s Supreme Court found that it was essentially a reincarnation of the banned neo-Nazi party Golden Dawn.

In its decision about which parties can run in the European Parliament elections, issued on Wednesday, the court found that the party, Spartans, “offered their party as a cloak for the new political party of Ilias Kasidiaris,” the former spokesman of Golden Dawn who is currently in prison.

Mr. Kasidiaris, the court said, is the true leader of Spartans, which “substitutes” for Golden Dawn, serving as a front that allowed him to circumvent eligibility restrictions.

The decision was announced a few weeks after a prosecutor for the Greek Supreme Court indicted several Spartans lawmakers on charges of electoral fraud, and before an anticipated ruling from another court that could lead to the party losing its seats in the Greek Parliament.

The decision comes as mainstream parties across the 27 nations that make up the European Union are increasingly concerned about the apparent growth of far and extreme-right movements, some of which looked to Golden Dawn’s success a decade ago for inspiration.

The E.U.-wide elections, scheduled to take place between June 6 and 9, will determine the more than 700 lawmakers in the next European Parliament. The institution has seen its clout grow in recent years as it blocked legislation and emerged as a forum for high-profile debates.

E.U. elections tend to be a kind of mass barometer on the mood of millions of Europeans and how they feel about their leaders. Most recent polls show that mainstream conservatives will dominate the new house, followed by mainstream social democrats and liberals.

But the polls also point to an increase in power among far-right and extreme-right parties, which represent an ideological coalition that is at its core nativist, anti-migrant, and sometimes expresses anti-European views in favor of powerful national governments.

In its decision on Wednesday, the Greek Supreme Court said that Spartans “aims at the weakening and demise of democratic political system, particularly through the use of violence or incitement to violence.” It also, the court said, embraced totalitarian ideologies, bigoted ideas, xenophobia and crimes against humanity.

Vassilis Stingas, the nominal leader of Spartans, was dismissive of the decision, noting that it was not based on a trial. “Because the Supreme Court said it, it’s the word of God?” he said. “I’ll accept, without complaint, any decision of the judiciary, as long as there’s a trial with evidence and witnesses. I don’t respect this decision.”

Mr. Kasidiaris and other Golden Dawn leaders were found guilty in October 2020 of running a criminal organization that attacked leftist critics and migrants, and the party, which shot to prominence in 2012 during the economic crisis in Greece, was disbanded.

Since then Mr. Kasidiaris, who is serving a 13.5-year prison sentence, has campaigned from his cell in a bid to re-enter Greek politics. Last year, the Greek government moved to block from Parliament the National Party-Greeks, which was founded by Mr. Kasidiaris, by pushing through legislation that barred from the legislature parties whose leaders have been convicted of serious crimes.

Α few months later, Mr. Kasidiaris publicly backed the then-obscure Spartans, and the party won 12 seats in the 300-seat Parliament, clearing the way for a legal fight on two fronts.

A special court is expected to rule before the European Parliament elections on complaints from citizens that Spartans engaged in electoral fraud. If Spartans loses its case, its seats would most likely either be redistributed across other parties or new elections would be held in those districts held by Spartans.

Separately, a Supreme Court prosecutor this month indicted 11 of the 12 lawmakers who were elected to Parliament under the Spartans banner, along with Mr. Kasidiaris. Five of those lawmakers are no longer affiliated with any party, and that case has not gone to trial.

Mr. Stingas has not been charged, and the Supreme Court suggested in its decision on Wednesday that he was little more than a figurehead. “The truth is that they were chosen by Ilias Kasidiaris,” the court said on Wednesday, “and not by Vassilis Stingas.”

Nikos Alivizatos, a prominent expert in constitutional law who was grabbed by the lapels and verbally assaulted by members and supporters of Golden Dawn in 2010, said the court’s reasoning appeared sound, but he also expressed concerns about how the law might be interpreted in the future.

“It’s one thing banning a convict from running,” he said. “It’s quite another to ban his friends and supporters.”

Matina Stevis-Gridneff contributed reporting from Brussels.