Jose Luis Rodriguez Zapatero, Spain's former prime minister, says passing a gay marriage law is his proudest achievement.

Mariano Rajoy and his conservative Popular Party (PP) won control of Parliament in a landslide victory last month as Spain shifted hard to the right while struggling under piles of debt and sky-high unemployment.

The power shift could be the undoing of the nation's gay marriage law, which was approved during the tenure of the Socialist Zapatero. Rajoy has suggested he would “change” the law and his party has filed a legal challenge based on technical grounds.

In an interview with Diario de Leon, Zapatero was asked what was his proudest achievement.

“If I consider the degree of recognition and gratitude I have received, then I think the gay marriage law. Not a week goes by without someone reminding me, thanking me … Yes, it's a decision that has left its mark,” Zapatero answered.

Pope Benedict XVI had previously urged Roman Catholics to oppose Zapatero's Socialist government and its liberal reforms, which include fast-track divorce and easier access to abortion.