The leader of Spain's Podemos party, which came third in Sunday's general election, said on Monday he would not allow the ruling People's Party (PP) to form a new government.
"So that no doubt remains... Podemos would not either actively or passively permit a PP government," Pablo Iglesias told a news conference.
The PP won the most seats - 123 - at Sunday's general election but fell far short of a majority. Podemos won 69 seats, and the Socialist Party (PSOE) won 90 seats.
Mr. Iglesias also said he would soon begin a round of talks with all other political parties to discuss possible agreements.