His Socialist party (PSOE) gained the most seats in April's election, but fell short of a majority.

Mr Sánchez now requires the backing of the left-wing Podemos party and other smaller parties to continue to govern.

If he fails, he has two more months as caretaker to find a solution or face another general election.

It will be the country's fourth in four years.

The Socialist leader lost his first post-election confidence vote on Tuesday, and his party has since been locked in negotiations to secure a new deal with representatives from Podemos and other parties.

What happened with the talks?

Talks on Wednesday appeared to have broken down.

Mr Sánchez's Socialists described Podemos's demands for key posts within government as "unacceptable", while Podemos said it had only been offered "inexistent or empty ministries".

The Socialist party's acting Deputy Prime Minister Carmen Calvo said Podemos "have literally asked us to give them the government" rather than form a coalition together.

A number of smaller parties said they would back Mr Sánchez, but only if a deal is agreed with Podemos. Spain's right-wing parties, meanwhile, have said they will not support him.

While Tuesday's vote required Mr Sánchez to secure an absolute majority of 176 seats in Congress's 350-seat parliament, Thursday's ballot scheduled for 14:25 (12:25 GMT) requires a simple majority of support from lawmakers. However, he only won 124 votes in the initial ballot, whereas 170 MPs voted against him and Podemos abstained.

Parliamentary backing for his coalition would bring weeks of fractious negotiations and political turmoil to an end, giving Mr Sánchez his first full term as prime minister.

What are the sticking points?

In return for joining a coalition, Podemos has asked for key positions such as deputy prime minister and social roles in his cabinet in charge of the environment, equality, employment and economics.

But after last-ditch talks on Wednesday, Podemos warned against raising expectations as there had been "little progress in the PSOE's proposals", arguing that the Socialists wanted to keep social affairs ministries to themselves. The prime minister insisted he had made a good offer.

 Mr Iglesias appeared exasperated during a debate in parliament earlier this week

Podemos leader Pablo Iglesias has already agreed not to seek a ministerial position himself, in an attempt to find a deal.

The two leaders have tried to find common ground for the past three months and tensions spilled into the open during a debate at the start of the week, when Mr Iglesias accused the caretaker prime minister of offering only "cosmetic" roles.

Separately, Mr Sánchez has said that Mr Iglesias, whose views on the question of Catalonia's independence starkly differ from his, is "the principal obstacle" to agreeing on a new coalition.

Among the smaller parties with representatives voting in parliament on Thursday are the conservative Popular Party (PP), the right-of-centre Ciudadanos (Citizens), the centre-left separatist Catalan Republican Left (ERC) party and the far-right Vox.

Why is Catalonia so important?

The future of Spain's semi-autonomous north-eastern region has been politically explosive ever since Catalonia's leaders held an independence referendum in October 2017. The vote had been outlawed by Spain, but the separatist leadership went ahead with it and then declared independence from Spain.

The issue was a key reason why this year's snap election took place, because Catalan separatists withdrew their support at a key moment in February. A dozen Catalan leaders went on trial in Madrid the same month, facing charges including rebellion and sedition, and separatist MPs wanted concessions from the government.

Mr Sánchez has sought to lower tensions with the region and has faced criticism from other political opponents.

His administration has engaged with the pro-independence government, holding talks with current Catalan president Quim Torra - prompting PP leader Pablo Casado to label him "the biggest villain in Spain's democratic history".

Ciudadanos has also accused Mr Sánchez of siding with "enemies of Spain" and wanting to "liquidate" the country.