Spain's prime minister criticized Thursday the main opposition party's proposed withdrawal of support for a large-scale multilateral trade deal between the European Union and Canada and expressed concern that his minority government might struggle to ratify the agreement in parliament.

The Spanish Socialist Party (PSOE) president, Cristina Narbona, signaled late Tuesday that her party would no longer back the Comprehensive Economic and Trade Agreement (CETA), a deal green-lighted by the European Parliament that was awaiting approval from EU national parliaments.

The PSOE's apparent change of tack, yet to be officially voted on at an upcoming party meeting, threatened the agenda of the governing Popular Party leader Mariano Rajoy, who criticized the decision, saying: "I simply believe that it is an error."

"The treaty is enormously positive," Rajoy added. "We supported it and so has PSOE, all European governments support it," he said.

"It can't be Spain that vetoes it," he added.

Rajoy's main hurdle is his government's minority in the nation's lower chamber of Parliament.

PSOE, Unidos Podemos (United We Can) and sympathetic regional parties in the opposition have the combined power to derail CETA when it comes to a vote.

"If PSOE and Podemos do not vote for it, I will try to find deals with others," Rajoy said upon his arrival in Brussels where members of the European Parliament's center-right group the European People's Party were gathered for a summit.

Earlier, Spain's Foreign Minister Alfonso Dastis accused PSOE of becoming populist and warned against economic protectionism.

The secretary-general of Citizens, Miguel Gutiérrez, earlier accused PSOE of "Podemosizing" itself.

Rajoy called the PSOE to use common sense, insisting the move would be damaging to both Spain and the Socialists.