"There is insufficient trust in the Alliance of Alenka Bratušek", prime minister-designate Miro Cerar said after the governing council of his party opted against a four-way coalition in a unanimous vote.

He said the party had concerns about her resignation as prime minister, which caused the collapse of the government, and the selection of EU commissioner candidates.

Cerar had indicated on several occasions that Bratušek's role in the new coalition was uncertain due to the way she was nominated for the post of Slovenia's EU commissioner, a procedure that it now being investigated by the Commission for the Prevention of Corruption.

While this leaves him with only two potential partners, Cerar was adamant this was a decision the party would stick with even if the SocDems decide on Thursday not to join the coalition.

"We are determined to put together a firm, good government that Slovenia needs. If others refuse to participate in that, we will have an early election," he said.

Bringing Bratušek into the fold was seen by analysts as a good way to keep in check potential extortion by the smaller partners, in particular the DeSUS, which has often resorted to the tactic in previous governments.

Cerar said it "often happened that a party makes its stay in coalition conditional", but the SMC will not yield to "unreasonable and senseless ultimatums".

Reacting to the news, Bratušek told the STA in a written statement she was "disappointed" and that the decision would turn out to be "irresponsible and bad for the country".

She called the commissioner appointment argument "unconvincing" and noted that DeSUS was also on the list of Slovenia's commissioner candidates, as is the vice-president of the SD, Tanja Fajon.

But Cerar insists there are differences, noting that Bratušek's was a case of "self-candidacy". He said his government would write to European Commission President-elect Jean Claude Juncker to clarify the situation.

Cerar said he was not disappointed by having a slimmer majority in parliament, noting that his party had done "everything it could" to build a firm government.