The decision was reached by the leaders of the Marjan Šarec List (LMŠ), Social Democrats (SD), Modern Centre Party (SMC), Alenka Bratušek Party (SAB) and the Pensioners' Party (DeSUS), who continue talks on Tuesday after the NSi walked out last week fearing that such a coalition would be too unstable.
Šarec said a tie-up with the NSi remained an option. "If it is not, we will look for another option," he said, noting that on substance significant headway had been achieved with NSi.
Only if the NSi declines will the quintet talk to the Left. "It is not fair to talk to both parties at the same time," he said.
Talks with the NSi had been at an advanced stage and the party was seen as having achieved a lot, which it admitted itself by citing fear of instability rather than programming differences as the reason to quit the talks.
A coalition with the Left is seen as potentially more difficult, as the party is far to the left of the quintet on issues as fundamental as economic policy and NATO membership.
One option mentioned by analysts - and previously not rejected by the Left - would be a minority government of the five parties with the tacit support of the Left in exchange for the realization of several projects that the Left holds dear.
Šarec, however, indicated that a minority government was not in the cards at this point when he said the goal was to get a PM designate confirmed in the second round of the nomination process, which requires an absolute majority in the National Assembly.
"We contested an election, everyone got their share of the vote and of responsibility," he said.
Luka Mesec, the president of the Left, described today's decision as "unserious and irresponsible" to the voters, who "voted for these five parties because they wanted a centre-left government."
"This quintet, however, is facing the citizens with the fact that the only option left is a rightist government with or without Janez Janša," he said.
Despite that, Mesec would not specify whether or not the Left was still willing to talk to the five parties.