Failure to get the necessary backing in the parliamentary vote would take Spain a step closer to holding its fourth elections in as many years.

Representatives from both parties have been trying to secure a deal for what would be Spain's first post-dictatorship coalition government following an inconclusive April general election.

But late on Wednesday, talks stalled.

On Thursday, Spain's Deputy Prime Minister Carmen Calvo said Podemos's demands for government posts were "unrealistic," accusing the far-left party of wanting "a parallel government" of its own.

Pablo Echenique, Podemos's chief negotiator, retorted that the Socialist party (PSOE) merely wanted to give them "a decorative role, which sounds good but that has few real responsibilities to better people's lives."

It looks unlikely that both sides will meet again as the clock ticks down to a parliamentary session that kicks off at 1.30 pm local time (1130 GMT), during which lawmakers will cast their ballot in a confidence vote for Sanchez.

Without the support of Podemos, which with its partner Izquierda Unida (United Left) has 42 lawmakers, Sanchez won't win the vote.

- Clash over ministries -

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The socialist premier came first in the April national poll but fell short of a majority with just 123 parliamentary seats out of 350, forcing him to seek backing elsewhere.

A spattering of regional parties, including a Catalan separatist grouping, have pledged their backing in Thursday's vote -- whether voting for him or abstaining -- but only if he seals a deal with Podemos.

Right-wing parties, meanwhile, have already said they won't back him.

If Sanchez loses the vote he will have another two months to find ways of getting support, either for a minority or coalition government.

Absent a deal Spain would have to go back to the polls in November.

Podemos has said it "doesn't want to enter government at any price".

The party has said it made several concessions already.

Its leader Pablo Iglesias, who does not get along with Sanchez, agreed last week not to be part of the government so as to unblock the situation.

But still Podemos has accused the socialists of refusing to give them positions that carry any weight.

The socialists revealed Podemos had asked for the post of deputy prime minister with responsibility for social rights and the environment, as well as five ministries including labour and fiscal justice.

"They literally asked us for the government," Calvo told Spanish radio.

"What would be left for the Socialist party to carry out its electoral programme?" she asked.

Sanchez's party, meanwhile, offered Podemos the housing, health and equality ministries.

But Echenique retorted these came with conditions.

"They made it clear that this housing ministry... wouldn't have the responsibility to stop evictions without alternative housing or regulate rental prices."