European leaders must confront separatism openly, and they must do so on political and moral grounds. Of course, they should play the legal card, invoking the EU treaties to remind the advocates of secession how hard it is to be accepted into the union and how easy to be expelled. This is what José Manuel Barroso, Commission president, skilfully did this week when questioned on September’s Scottish independence referendum, to the indignation of Alex Salmond, Scotland’s first minister. But it is not enough. European leaders must unmask the hypocrisy of nationalism.

Brussels, however, has coined a catchphrase behind which its spokespeople take refuge: Catalonia is a Spanish “internal matter” on which European institutions have no comment. Mr Barroso himself has used this formula several times, including in his reply to a letter from Artur Mas, president of the Catalan government, urging European leaders to support a referendum in November on secession. From Paris to Berlin, reactions have been equally non-committal.

A shrewd political strategy? A clever way of snubbing the separatists to reassert the sovereignty of central government? Many, including in Madrid, argue that this is the case. The government of Mariano Rajoy, prime minister, does indeed have the political legitimacy and legal instruments to counter successfully one of the most serious challenges yet to Spain’s laws and liberties. But any attempt to shrug off intended secession as an “internal matter” is a mistake.

In Britain, the strident euroscepticism of the UK Independence party obscures the fact that Scottish nationalism also contradicts basic European principles, values that should be defended in Spain, too. Catalan separatism runs contrary to the values that define Spain as a democracy and the EU as a feat of civilisation. It puts identity before citizenship and defies the basic idea upon which our peace is founded, that Europe should not be the segregationist sum of ethnic or cultural nations but a union of democratic states. In this, if not in their historical or constitutional underpinnings, the plans for Scottish independence and Catalan secession look and sound worryingly alike. They should be confronted on the same grounds. The separatists hail the EU as a model of integration but work tirelessly to disintegrate one of its member states. Mr Mas and his supporters urge Europe to support Catalonian independence in the name of democracy (“Let us vote!”) but simultaneously undermine democracy in Spain in the name of identity.

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How? They distort historical facts to justify imaginary grievances: they have transformed commemorations of the end of the War of the Spanish Succession in 1714 into a denunciation of 300 years of “Spain against Catalonia”. They challenge the rule of law and, with it, the foundations of peaceful coexistence among Spaniards. The Catalan government has refused to abide by court rulings requiring children to be taught a minimum number of lessons in Spanish.

Secessionists often talk as though they speak for everyone in Catalan society. They do not. Catalonia is home to a diversity of views. When last week a Barcelona-based group of German businessmen warned of the economic impact of secession, prominent figures in the separatist camp were quick to dismiss them in shockingly pejorative terms – for which they later had to apologise.

Voters across the continent are demanding clarity. They want to know what Europe stands for and why it is worth their solidarity and sacrifices. In the memory of 1914 they may find the answer: there is nothing nobler, more decent or more necessary than the defence of individual rights and liberties against the smothering dreams of nationalism. Today, there is no room for ambiguity or silence. The issue of separatism is a profoundly European matter, putting to the test what the union is and what it stands for – and it demands a clear response.

The writer is an MP for the ruling Popular party in Spain