Spain's King Felipe VI has said he would not put forward a new candidate to seek the confidence of parliament to become prime minister, a move that paves the way for a new general election in November.
The king said in a statement that none of the political leaders he met with over the past two days had enough support to secure a parliamentary majority.
Acting Prime Minister Pedro Sanchez's Socialists won an April general election but only secured 123 of the parliament's 350 seats.
That left him dependent on support from other parties in Spain's deeply fragmented parliament to be confirmed premier for another term.
However his talks with other parties to try to win their support have so far been fruitless and if the assembly does not confirm Mr Sanchez by Monday, fresh elections will automatically be called for 10 November - the country's fourth poll in four years.
Spain, the eurozone's fourth-largest economy, has been gripped by political instability since the December 2015 elections ended the traditional two-party system with the emergence of far-left group Podemos and the business-friendly Ciudadanos.
The recent rise of far-right party Vox, which entered parliament following April's election, has further complicated the political picture.