Nigel Farage, the former Ukip leader, predicted an independent Catalonia would prove to be a far bigger existential threat to the EU than Brexit.
The MEP claimed the anti-Brussels strand of the Catalonian independence movement had been massively underestimated.
Speaking during his LBC radio show, he said unlike Scotland, Catalonia is as much against the leaders of the EU as they are against those in Madrid.
Mr Farage said the campaign for Scottish independence had been received much more favourably by the EU than the Catalan movement.
He said officials in Brussels wanted Scotland to break away, largely because Alex Salmond was a “fanatical” supporter of the European project.
Unlike the Scots, the MEP revealed Catalans consider the EU chiefs as much of an enemy to their cause as the central government in Madrid.
He told listeners: “This is Juncker’s worst nightmare. It is pretty clear that the Catalans are as much against the leaders of the EU as they are against the leadership from Madrid.
“This is an EU where cracks are already starting to show, as Juncker has admitted.
“Catalonia breaking away would make Brexit look like a Sunday afternoon picnic.
“The EU wanted Scotland to leave the UK because Salmond was so fantastically in support of the EU and wanted Scotland to join the euro.
“But, Catalonia, they pose a mortal threat to Brussels.”
He also blasted the European Union’s response to the declaration of independence.
Mr Farage explained: “Those in Brussels, they put European integration above any human rights.
“The way the international community have ganged up and tried to crush them is monstrous.
“I mean, Frans Timmermans comments, and remember he is Juncker’s number two in the European Commission, said the rule of law was appropriate, and necessary force was used, which was absolutely monstrous to say.”