Spain’s Socialists (PSOE) have rejected a demand from the anti-austerity Podemos party for exclusive talks to form a new government.
The country remains in a political stalemate following December’s inconclusive election.
On the request of King Felipe VI, PSOE leader Felipe Sanchez has been leading talks with other parties.
But Podemos leader Pablo Iglesias demanded that he stop speaking with the centrist Ciudadanos.
“Our negotiating team will be ready as soon as the Socialists opt for a government of progress,” Reuters quoted him as saying.
The move followed a survey on Thursday that showed Podemos would come second in a new national election, overtaking the Socialists.
Despite coming first in the election with close to 29% of the vote, acting Prime Minister Mariano Rajoy of the conservative Popular Party (PP) lacks the support to form a government.
Mr Sanchez said he would continue to talk with all parties except the PP and called for Mr Iglesias to reconsider his demand.
The El Pais newspaper quoted him as saying: “There are things that divide us, but one thing unites us all: we have to end the government of Mariano Rajoy and the PP.”
Mr Sanchez earlier pledged to form a “progressive and reformist” government.
Podemos, Ciudadanos and smaller regional parties hold the balance of power in Spanish politics, which has been dominated for decades by the PP and PSOE.
If no working government can be formed, a fresh general election will be called.