Belgium MEP Philippe Lamberts, the head of the Green grouping in the European Parliament, said the crisis “threatened the spirit of European integration, even more than Brexit”.
His warning comes after Catalonia’s president, Carles Puigdemont, announced that the region will declare independence next week, as tensions soar.
European Commission vice president Frans Timmermans has also waded in to slam the region for its attempt at independence.
He said: “There is a general consensus that the regional government of Catalonia has chosen to ignore the law when organising the referendum.”
Mr Timmermans was referring to the disputed independence vote on Sunday in Catalonia, which the Spanish government in Madrid says violates the constitution.
Spanish police left more than 900 voters and others injured as they tried to stop the vote, and say more than 400 police were left with bruises.
Violence spilled over into the street again yesterday, as Spanish loyalists and Catalan separatists clashed in Barcelona in the upmarket Sarrià-Saint Gervasi neighbourhood.
In a statement Wednesday to the European parliament, Mr Timmermans said members of the bloc must “respect for the rule of law is not optional, it is fundamental”.
Mr Timmermans also said “it is fundamental that the constitution of every one of our member states are upheld and respected”.
Meanwhile, Catalonian leader Mr Puigdemont said on Wednesday he was in favour of mediation to find a way out of the Catalan crisis but that the Spanish government had not accepted it.
He said: ”This moment calls for mediation. We have received various offers in the last hours and we will receive more. All of them know I am ready to start a mediation process.
“I will repeat it as many times as necessary: dialogue and agreement are part of the political culture of our people. However, the state has not given any positive answer to those offers.”
The Catalan parliament is expected to unilaterally declare independence from Spain next Monday after a referendum that the Spanish government and courts have declared illegal.
Mr Puigdemont added: ”I am sure that in the next few days we will show the best of our country when the institutions of Catalonia will have to apply the results of the referendum.”
In an interview with German newspaper Bild, Mr Puigdemont said: “I already feel as a president of a free country.”
Yesterday, Spain’s King Felipe VI said he was committed to the unity of Spain as he accused Catalan leaders of shattering democratic principles and of dividing Catalan society.
In a televised address to the nation, the king said the “irresponsible behaviour” of the Catalan leaders had undermined social harmony in Catalonia.
“Today Catalan society is fractured and in conflict,” he said.
He said the Spanish crown was strongly committed to the Spanish constitution and to democracy and underlined his commitment as king “to the unity and permanence of Spain.”