Sanchez, who won the most seats but fell short of a majority in a parliamentary election in April, faces a vote in parliament next week on whether to swear him in, but still lacks the support he would need from other major parties to be confirmed as prime minister.

If he loses the vote, a two-month countdown will begin until a new election is held.

"I shouldn't be the excuse for the Socialists to justify why there will be no coalition of the left. Whether I'm in the cabinet or not will not be a problem as long as there are no more vetos and Podemos' presence in the government reflects the votes we have won," party leader Pablo Iglesias said on Twitter.

Podemos is the most natural political ally for the Socialists but the parties have failed to reach a deal. Sanchez opened the door on Thursday to Iglesias' demand to include Podemos lawmakers in a coalition government, but with the condition that Iglesias himself would not be among them.