A row has broken out among political parties in Europe after the centre right group to which Ursula von der Leyen belongs refused to sign a letter condemning the far right and a spate of attacks on politicians in Germany, Belgium, Spain, Sweden and Ireland.

The open letter calls on von der Leyen to “reject any normalisation, cooperation or alliance with the far right and radical parties” and denounces a physical assault on the German centre-left politician Matthias Ecke. Ecke was “seriously injured” after allegedly being attacked by four young men while putting up campaign posters.

The letter has been signed by five of the main political parties in the European parliament, including the Progressive Alliance of Socialists and Democrats, Renew, representing the liberals, the Greens and the Left.

But the European People’s party (EPP), the largest group in the parliament representing conservative-leaning parties throughout Europe, declined.

The French MEP Valérie Hayer, the head of Renew, said her group, which is backed by Emmanuel Macron, regretted that the EPP had not signed. She said: “This puts their commitment to the common fight against destructive, far-right forces into question. We urge the EPP to reconsider and to join this pro-European commitment.”

Tensions have been building since a leaders’ debate in April over which alliances the mainstream parties are prepared to form after the European elections in June. Von der Leyen, who is running for a second term as president of the European Commission, was guarded at the debate about closing down alliances with the rightwing ECR group, which includes the party of the Italian prime minister, Georgia Meloni, as well as extreme right parties such as Éric Zemmour’s Reconquête in France, Spain’s Vox and the Sweden Democrats.

At the time, Von der Leyen criticised the far-right Identity and Democracy group which unites France’s National Rally, Alternative für Deutschland (AfD) and the Danish People’s party, condemning them as “Russian proxies”, but her more equivocal position on the ECR group drew fire from critics.

The EPP reacted furiously to the open letter row, saying the political parties that had signed were playing a “nasty” game, and suggestions that von der Leyen would work with the far right were “bullshit” and “fake news”.

Von der Leyen had frequently ruled out working with the far right, the EPP added.

On Wednesday, von der Leyen restated her intention not to work with the far right. In a speech in Germany, she said she would only work with parties that had “a clear commitment to the rule of law, a clear commitment to Ukraine and against the crimes of the Kremlin, and a clear commitment to our Europe.

She said: “With the servants of the Kremlin, despisers of democracy and extremists, there is no state and no European Union either.”

Sources said the EPP had balked at the use of the term “far right” in the letter. It is understood the EPP had wanted the letter to condemn the “far left” and to add a paragraph restating that the group of parties would only, as the EPP has declared, work with parties that are pro-Ukraine, pro-Nato and pro-rule of law.

The signatories of the letter wrote: “Once more the far right is attempting to bring back the darkest pages of our history, challenging everything we built and poisoning our democracies.”

They went on to “strongly condemn the constantly growing [number of] cases of harassment, vandalism, spread of disinformation, defamation and hate speech by far right parties” across all member states.

The EPP leader in the European parliament, the MEP Manfred Weber, said: “Our principles are clear: we only cooperate with parties that are pro-European, pro-Ukrainian and pro-rule of law.”

Attacks on politicians on social media are now commonplace across Europe but there is mounting concern over physical assaults and harassment at their private homes.

The motive for the attack on Ecke on Friday remains unclear. None of the teenagers has so far spoken, according to police, who have said they are being investigated for grievous bodily harm. But Nancy Faeser, the federal interior minister, has blamed the far right for stoking a climate of violence with hate-filled rhetoric, including on social media.