Prime Minister Mariano Rajoy is likely to be forced out of office after a key party said it would support a motion of no confidence against him.
Spanish Prime Minister Mariano Rajoy attends a debate on a no-confidence motion. Photo: AFP
The no-confidence vote is scheduled to take place on Friday (local time) and was tabled by the opposition Socialist party.
Socialist leader Pedro Sánchez needs a majority of 176 votes to become leader.
The Basque Nationalist Party (PNV) has said that it will support the motion, which may then propel Mr Sánchez into office.
The party holds a crucial five seats in parliament and announced its intentions on Thursday.
Mr Sánchez has called on Mr Rajoy to step down ahead of the vote. But a senior official from Mr Rajoy's People's Party (PP) insisted the prime minister would not resign.
Mr Sánchez tabled the motion of no confidence after the PP was implicated in a huge corruption scandal.
He said Mr Rajoy, 63, had failed to take responsibility for his party's involvement, which came to light last week after one of its former treasurers was given a 33-year jail sentence.
The High Court in Madrid convicted Luis Bárcenas of receiving bribes, money laundering and tax crimes.
The case centred on a secret campaign fund the PP ran from 1999 until 2005.
Many Spanish voters, exasperated by corruption scandals involving the traditional centre-right PP and centre-left Socialist parties, have abandoned them for newcomers like the left-wing Podemos (We Can) and centrist Ciudadanos (Citizens), as well as regional parties.
Could Mariano Rajoy step down?
"Resign, Mr Rajoy, your time is up. Resign and this no-confidence motion ends here, today and now," Mr Sánchez said during the two-day debate that began in parliament on Thursday.
"Staying on as prime minister is harmful and is a burden not only for Spain but also for your party."
Mr Rajoy insisted he would stay put and accused the Socialists of opportunism.
"Mr Sánchez is concerned because he is not doing any better in the polls and he has realised that he will never reach government through elections," he said.
The PNV later confirmed that it would support the no-confidence motion, effectively forcing Mr Rajoy from office.
"We believe we are responding to what most Basques want and best complying with our responsibility by voting yes (to the motion)," party spokesman Aitor Esteban said.
Mr Rajoy currently has the support of the PP, Ciudadanos and two regional parties, which between them can only muster 169 votes.
The PM did not return to parliament for the Thursday afternoon session of the debate, raising further speculation about his intentions. However, PP secretary-general Maria Dolores de Cospeda later told journalists that "Mariano Rajoy will not resign".
Correspondents say Mr Rajoy's resignation could be to the advantage of the government as it would usher in a caretaker period of weeks, possibly months, before a new PM is sworn in.
If he loses Friday's vote, however, Mr Sánchez is likely to replace him immediately.
- BBC