Madrid (AFP) - Spanish caretaker prime minister Pedro Sanchez lost a first parliamentary confidence vote on Tuesday as he seeks to remain in power after an inconclusive general election.

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.

If he manages to form a coalition government, it would be the first in post-dictatorship Spain. 

If he fails to reach an agreement, Spain will hold its fourth elections in as many years.

A total of 124 lawmakers in the 350-seat parliament voted for the Socialist premier in Tuesday's vote, leaving him well short of the absolute majority he needed.

Sanchez is currently caretaker premier after coming first in the April general election but without the majority he needed with just 123 seats, forcing him to look for support.

But Podemos and regional parties that could back Sanchez have lashed out, accusing him of not properly reaching out to possible allies despite needing their help.

'Playing poker'

The second vote on Thursday requires only a simple majority.

With the support of 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 (PSOE) has been locked in negotiations with Podemos for months and only recently reluctantly agreed to form a coalition government with them.

But Podemos leader Pablo Iglesias accused the socialists of refusing to give his party positions that carry any kind of weight and wanting them to be "mere decor" in the government, in a parliamentary debate on Monday.

"We have to talk about content, we have to talk about programmes, and not so much about who will occupy the blue seats," Sanchez retorted on Tuesday, in reference to the places reserved for the cabinet in parliament.

In the end, Podemos lawmakers abstained in the vote.

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

It also slammed him for not having mentioned the separatist crisis in Catalonia in his Monday speech to parliament.

ERC had previously said it would not stand in Sanchez's way despite their differences over how to handle the crisis, which culminated in a failed declaration of independence in October 2017.

But Gabriel Rufian, ERC's leader in parliament, said "the feeling was that you are playing poker with the hopes of hundreds of thousands of people who came out to vote on April 28."

ERC voted against Sanchez Tuesday.

Aitor Esteban of the PNV Basque nationalist party said the socialists had not even been in touch with them in the past few weeks.

"They have taken for granted that our vote was going to be positive," he said.

His party also abstained in Tuesday's vote.

If Sanchez cannot secure the votes he needs on Thursday, he has another two months to find a solution, failing which the Spanish will face another general election.

"While minority governments are a common occurrence in Spain, coalition governments are not, which explains PSOE's reluctance to share power," said Federico Santi, Spain analyst at consultancy Eurasia Group.

"This is the latest sign of the many outstanding issues that remain to be addressed and continued animosity and lack of trust between the two parties. 

"However, the uncertainty around new elections, and pressure from both parties' base to avoid an electoral repetition suggests an agreement is ultimately likely, if not this week than later this summer."