However, if the situation changed and the NSi were excluded from the potential coalition, the council could decide on the party's cooperation with Šarec again, he told the press on Tuesday.
The Left decided against joining the emerging Šarec-led coalition because it cannot support "the kind of government that is being formed right now", Mesec said.
He believes the centrist parties have missed the opportunity to form a left-leaning government by excluding the Left from the talks.
Šarec is currently conducting talks on a future coalition with the SocDems, the Modern Centre Party (SMC), the Alenka Bratušek Party (SAB), the Pensioners' Party (DeSUS), and the centre-right NSi.
Mesec believes the substantive differences between the Left and the emerging coalition are too big.
"We wanted a government that would follow the guidelines we had presented before the election," he said, pointing to a notable and permanent rise of wages and pensions, addressing the issue of precarious work forms, an environmental reform, curbing of privatisation and a housing programme.
"Unfortunately, we'll get a government that will, as it seems, again let the favourable economic trends go to waste, give tax breaks to the rich and the capital, continue to sell state assets, primarily the NLB bank, and that will continue to degrade the health system ...
"Such a government cannot have our support, because it moved even further to the right than the previous government of Miro Cerar."
Mesec denied having presented impossible demands to Šarec. "The truth is that the Left entered the negotiations completely honestly and openly, without any demands beforehand.
"We proposed that we find common ground or compromises where we don't see eye to eye. But the centre excluded us from the talks before any substantial negotiations even started."
He said that the Left had learned about the content of the draft coalition agreement from the media.
As for their proposal of a referendum on Slovenia's leaving NATO, Mesec said this would not be their ultimatum.
However, the Left would have insisted on cancelling the planned investment into the forming of two battalion battle groups in the amount of EUR 1.2bn.
Mesec noted that the first draft coalition agreement had suggested there was readiness for talks in this direction or for at least lowering the amount of money to be spent.