Leading left-wing parties across Europe have ruled out alliances with the far right and pledged to “relentlessly combat hatred, racism and xenophobia” ahead of European parliamentary elections likely to see significant gains by hardline nationalists.

“Turbulent times require a clear course and a firm attitude. They do not tolerate vagueness or cowardice,” said the joint appeal, published on Thursday and shared with the Guardian. “The time has come to become democrats of combat, no longer of habit or comfort.”

Signatories included MEP Raphaël Glucksmann, who heads the French Socialists’ list for the June election, Olivier Faure, the party’s first secretary, and Frans Timmermans, a former European Commission vice-president and leading member of the Dutch Labour party (PvdA).

Others to join the appeal were Paul Magnette of the Belgian Socialist party, Nicolas Schmit, lead candidate of Europe’s Socialists & Democrats (S&D) for the job of European Commission president, the Spanish Socialist Iratxe García, German Social Democrat Katarina Barley, Poland’s Robert Biedroń and Italy’s Elly Schlein.

“While the far right is progressing throughout Europe, we solemnly commit not to give up on our democratic, humanist and united principles,” the leftists’ statement said, pledging “to build a strong barrier against the far right” at all levels.

The signatories undertook to “reject any electoral or governmental alliance with far-right parties, at national or European level, and to immediately exclude any formation from our European social democratic family which contravenes this rule”.

It follows current Commission president Ursula von der Leyen’s refusal to say her centre-right group would not in future work with national-conservative and far right parties including Brothers of Italy, Poland’s Law & Justice (PiS) and Spain’s Vox.

Another trigger for the socialists’ intervention is thought to be the decision by centre-right Dutch VVD party last week to enter government with Geert Wilders’s far-right Freedom Party (PVV). As a result, the European parliament’s liberal group – to which the VVD belongs – has promised a vote on whether to allow it to remain a member.

Polling suggests the S&D group will again finish second in the European elections, perhaps shedding a handful of MEPs, with the main centre-right European People’s party (EPP) expected to remain the largest group.

However, far-right and hardline conservative parties are forecast to make significant gains, finishing first in nine countries including Austria, France and the Netherlands and second or third in nine more including Germany, Spain, Portugal and Sweden.

Analysts say those gains may make little immediate difference to the workings of the parliament, because its three mainstream groups are likely to retain a working majority of MEPs overall and the far right are divided by factional rivalries.

However, the combination of a stronger far-right presence in parliament and in several national governments around the continent could end up compromising key EU projects such as the green deal and recently agreed migration package.

Signatories to the leftwingers’ statement called on all parties supporting “the democratic principles at the foundation of European construction” to “escape ambiguity or compromise”, saying it was time to “defend our principles and our open societies with infinitely more vigour”.

Wherever the far right attacked minorities, they said, “We will be there. Wherever you attack women’s rights and LGBTQI+ rights: we will be there. Wherever you insult a European citizen to inferiorise [sic], humiliate or dehumanise them: we will be there.”

The signatories said they would stand up to Europe’s far right “without weakening or bending”.

“When a European citizen is attacked and humiliated because of who he is, it is the whole of Europe and, beyond that, humanity that is attacked and humiliated,” they said.