Spain's Socialist leader Pedro Sanchez failed on Tuesday in a first attempt to get parliament's backing to form a government, leaving him two days to try and strike a deal with the far-left Podemos ahead of a second vote.

Sanchez, who has been acting prime minister since an inconclusive election in April, needed an absolute majority of at least 176 votes in his favor in the 350-seat house to be confirmed as prime minister but failed to get it.

A total of 124 lawmakers in the 350-seat parliament voted for Sanchez, 170 against him, another 52 abstained with the remaining four votes belonging to suspended lawmakers.

A second, decisive vote has been scheduled for Thursday afternoon, before which Sanchez needs to reach a coalition deal with far-left Podemos, a party that was once an arch-rival.

The second vote on Thursday requires only a simple majority. 

With the support of far-left Podemos's 42 lawmakers, and a few others from small regional parties, he could get through.

But given the anger of these potential allies, that support looks uncertain. 

Sanchez's Socialist party has been locked negotiations with Podemos for months and only recently reluctantly agreed to form a coalition government with the party.

But Podemos leader Pablo Iglesias angrily lashed out at Sanchez in the parliamentary debate on Monday. 

He accused the socialists of refusing to give his party positions that carry any kind of weight and wanting them to be "a mere decor" in the government.

Catalan separatist party ERC, meanwhile, accused Sanchez of being "irresponsible" for not appearing to want to negotiate with anyone.

If Sanchez is not confirmed as prime minister on Thursday, further votes could be held in September. If that failed too, a repeat election would be held on November 10.

(With input from Reuters, AFP)