The lead candidate for Alternative für Deutschland (AfD) in the European parliamentary election has resigned from the German far-right party’s leadership, as growing divisions between Europe’s nationalist parties threaten to undermine their expected gains in next month’s ballot.

Maximilian Krah, who last weekend told the Italian newspaper La Repubblica that the SS, the Nazis’ main paramilitary force, were “not all criminals”, said in a statement on Wednesday that his comments were “being misused as a pretext to damage our party”.

His SS remark, the latest in a series of controversies involving Krah and AfD, this week prompted France’s far-right National Rally (RN) to say it would no longer sit in the same European parliamentary group as the German party after the June elections.

Marine Le Pen, RN’s leader, who has spent years trying to normalise her party to appeal to mainstream voters, confirmed on Wednesday that it needed to make a “clean break”, accusing AfD of being held hostage by its most radical elements.

“It was urgent to establish a cordon sanitaire,” Le Pen told French radio. “The AfD goes from provocation to provocation. Now it’s no longer time to distance ourselves – it’s time to make a clean break with this movement.”

The two parties currently dominate the European parliament’s radical right and Eurosceptic Identity and Democracy (ID) group, which also includes Matteo Salvini’s League in Italy, Austria’s Freedom party (FPÖ) and Vlaams Belang in Belgium.

The group’s members – many of which are still viewed as extreme in their national contexts – are on course to become the biggest winners of the European elections, with polls predicting their total seat tally could rise from 59 MEPs to about 85.

The national-conservative European Conservatives and Reformists (ECR) group, which includes Giorgia Meloni’s Brothers of Italy, Poland’s Law and Justice (PiS), Spain’s Vox, the Finns and the Sweden Democrats, is also expected to advance, to about 75 seats.

However, a major shake-up of the parliament’s rightwing and far-right groups now appears inevitable, with the formation of one or more new groups possible and little certainty as to how – or indeed whether – they will be able to work together.

AfD has shot up the polls to become Germany’s second most popular party this year, but its support has recently dropped by several percentage points amid intense scrutiny of its policies and the conduct of some of its leaders, including Krah.

The party faced mass street protests earlier this year after senior figures attended a meeting where the deportation of Germans with immigrant backgrounds was discussed, and over allegations that it harbours agents for Russia and China.

Last week, a German court ruled that domestic security services could continue to keep the AfD under surveillance as a potentially extremist party. The party denies all allegations of racism, dismissing criticism as politically motivated.

On Wednesday Krah, who is being personally investigated for alleged links to Russia and China, which he denies, said “the last thing that the party needs now is is a debate about me”.

He said it was essential that AfD maintained its unity in the run-up to the 6-9 June elections and that “for this reason, I will not be making any more campaign appearances and will be stepping down as a member of the federal committee”.

Polls suggest Le Pen’s RN, meanwhile, will easily win the European election in France, with the party’s list – headed by Jordan Bardella – set to receive 31% of the national vote, almost double the score of President Emmanuel Macron’s centrist coalition.

RN’s campaign manager, Alexandre Loubet, told local media on Tuesday that the party had had “frank discussions” with AfD, but that Bardella had taken the decision to split from the German party because “lessons have not been learned”.

Analysts said the move was clearly made with domestic politics in mind, with Le Pen, who scored 41% in the second round of France’s 2022 presidential vote, likely to have her best chance yet of capturing the Elysée in elections due in 2027.

“This shows Le Pen doesn’t care about EU or the game in Brussels,” said Mujtaba Rahman of the Eurasia Group political risk consultancy. “All she wants to do is polish her domestic image in France. It’s all about domestic politics.”

It does, however, leave the future of the far right’s collective influence in Europe in doubt. It is unclear whether RN plans to leave ID, or try to have AfD – which is on course to win 16 MEPs – excluded, or how other members of the group will respond.

Italy’s prime minister, Meloni, has repeatedly said she hopes to unite Europe’s various rightwing parties which, while agreeing on issues such as migration and fighting green legislation, are deeply split on others including their relationship with Russia.

ECR members are mostly populist, nationalist and conservative, and have been (or are) in government. They are EU-critical but generally seen as relatively constructive at the EU level, having frequently helped draft EU legislation, and back Ukraine.

ID members, by contrast, are mostly populist far-right, more Moscow-friendly, and EU-disruptive: AfD talks favourably of a “Dexit” referendum, while RN ’s proposed “national preference” in jobs and benefits are not compatible with the single market.

Ursula von der Leyen, the centre-right president of the European Commission, has not ruled out working with ECR after the elections, but would be unlikely to do so if the group included RN – still less AfD or other more radical ID members such as PVV.

Either way, said Cas Mudde, a populism and far right specialist at the University of Georgia, the RN-AfD split was “a big decision for RN and for Europe’s far right. It almost certainly will hurt their already fairly limited political power in Brussels … Politically, it could become a pyrrhic victory if the parties remain so divided.”