With the regional government in Barcelona - which has pencilled in a consultation on independence on November 9 - set to potentially give such a vote a green light on Friday, the spotlight could turn to Catalonia's drive to cut ties with Spain a day after Scots are due to vote on whether to stay in Britain.
"The whole of Europe believes these processes are enormously negative," Rajoy told parliament in Madrid, when asked whether his government would facilitate Scotland's integration into the European Union if it voted "yes" to independence on Thursday.
Read MoreScotland vote worries its No. 3 industry: Whisky
"They create big problems in areas such as currency, public debt, pension payments... they produce recessions and poverty for all," the Spanish leader said. "These processes are a torpedo below the waterline of the European spirit."
Follow us on Twitter: @CNBCWorld