Spain’s centre-right Ciudadanos party has warned Prime Minister Mariano Rajoy that the ruling party’s recent corruption conviction has fatally wounded the government and plunged the country into an “institutional crisis” that can be resolved only through a snap general election.

Speaking to The Guardian, the leader of the Ciudadanos party, Albert Rivera, said the government was “over and done with”.

Spain’s highest criminal court last week ruled the governing People’s party (PP) had benefited from long-running illegal kickbacks-for-contracts scheme involving one its former treasurers and ordered it to pay a €240,000 fine.

“This is the first time in Spanish democracy that a governing party has been convicted of corruption and that’s why we think this legislature is done for,” said Rivera.

“It’s a tremendous blow to the PP and to Rajoy, it’s done for the legislature and opened up an institutional crisis that can only be solved by letting Spaniards have their say by calling early elections.”

As reported by the Reuters news agency, Rivera said the prime minister’s situation was “untenable”, adding: “Rajoy needs to stop denying reality and admit this government has run its course to avoid any further damage to Spain.”

Despite the strong language, however, Ciudadanos is so far refusing to support the no-confidence motion, which was proposed by the Spanish socialist party (PSOE). The motion is only being backed by the anti-austerity Podemos party.

However, if Rajoy fails to call an immediate election, said Rivera, Ciudadanos would propose an instrumental motion that would see an “independent candidate” dissolve parliament and call new polls after obtaining final approval for Spain’s 2018 budget.

But such a move could prove tricky without the backing of another party as Ciudadanos has only 32 MPs. This is three short of the 35 needed to propose such a motion.

Meanwhile, recent opinion polls suggest that Rivera’s party, which was founded in Barcelona in 2006, would finish first in a general election, ahead of the PP, the PSOE and Podemos.

According to Reuters, Ciudadanos and Podemos’s performance in the 2015 general elections ended decades of two-party domination. Since then, the anti-austerity grouping has begun to stall, while Rivera’s party has gained momentum.