Independence referendums in Scotland or Spain's Catalonia region are like a torpedo to European integration, PM Mariano Rajoy has warned.
He told MPs such processes created "more economic recession and poverty".
Pro-independence Catalans are watching Scotland's referendum on independence on Thursday closely, with plans for a similar vote in Catalonia in November.
If Scotland backed independence, it would have to reapply to the EU as a new member state, Mr Rajoy said.
'Torpedo below the waterline'
Catalonia's regional parliament is set to vote on a resolution on Wednesday that could pave the way for a November referendum on independence from Spain.
Last Thursday, hundreds of thousands of Catalans formed a "V" for "vote" along two of Barcelona's main roads calling for their right to vote.
But Spain's national government is opposed to any move towards independence and its blessing is constitutionally required to make a referendum legal.
Speaking to members of parliament on Wednesday, Prime Minister Rajoy described referendums like the Scottish example as "a torpedo below the waterline for European integration".