One of the Spanish left’s most popular politicians has announced the creation of a splinter party that will run in the 10 November parliamentary elections, adding further uncertainty to the country’s fourth national ballot in four years.

Spain has not had a stable government for years, with mainstream parties and newcomers that have appeared over the past five years struggling to strike deals.

The Socialist leader, Pedro Sánchez, won a snap election in April but without enough seats for his party to govern on its own, and he ended up calling an election for November after failing to reach a deal with the leftwing Unidas Podemos.

Íñigo Errejón, a co-founder of Podemos, which he left in January after disagreements with its leader, Pablo Iglesias, said he was launching the party, Más País (More Country), because he wanted to help form a government and encourage disillusioned leftwing voters to turn up at the polling stations.

“I understand the widespread anger of the Spanish people with the current leaders and with the political stalemate,” he said in front of a crowd cheering “president”.

Errejón’s party could further fragment the vote and either help the right get a majority or make it harder for any bloc to do so. There was already no guarantee the new election would make it any easier to strike a deal to form a government, and there will now be three main parties on the left and the right, after the far-right Vox made a breakthrough in the April election.

“This makes it even more difficult to foresee how the new parliament will look, and what type of feasible majorities will emerge,” said José Fernández-Albertos, a political scientist at Spanish National Research Council.

Errejón, 35, who is considered more moderate than Iglesias and was his second in command in Podemos, had in 2016 backed an agreement with the Socialists that Iglesias eventually rejected. In a recent interview, Errejón said he was in favour of a government deal with Sánchez.

In his first speech as a candidate, Errejón said he wanted to achieve an agreement among leftwing forces to form a government.

Sánchez is currently ahead in opinion polls, but Errejón and Iglesias are on bad terms, making a three-way government deal potentially difficult.

Based on how a previous, regional party he set up earlier this year did in local elections in May, Errejón could hope to get as many as five seats in the national parliament in the Madrid region alone.

Some analysts said the new party could convince leftwing voters put off by the lack of a deal between the Socialists and Podemos to turn up at polling stations.

“The leftwing electorate is going to be motivated. Some to defend Iglesias, others to punish him,” Fernández-Albertos said.