The leader of Slovenia's anti-migrant SDS party, which won June's election, said Thursday he had so far failed to win the backing needed for a coalition government.
"I inform you that for the moment in parliament there is no possibility of forming a stable government", SDS leader Janez Jansa wrote in a letter to President Borut Pahor.
Pahor had given Jansa until Thursday to say whether he had been able to win a majority in parliament.
Pahor subsequently announced he would not be putting forward any candidate for prime minister.
That means that soon a second round of talks will have to begin in which parties in parliament will have an additional two weeks in which to put forward a candidate themselves.
If that in turn fails, Slovenian law provides for a last-ditch period of 48 hours for further talks before fresh elections have to be called.
Jansa's centre-right Slovenian Democratic Party (SDS) won 25 of the 90 seats in parliament in June's election.
Even taken together with its most likely coalition partner, the centre-right Nova Slovenija (NSi), which has seven seats, the SDS is well short of a majority.
Jansa has also made overtures to five centre-left parties, led by the anti-establishment LMS of comedian-turned-politician Marjan Sarec, which together hold 43 seats.
However, they have so far ruled out any co-operation with Jansa.
The Sarec-led bloc had been trying to broker its own deal to form a majority government with NSi but that failed when NSi pulled out, claiming a six-party coalition would be too unstable.
In his statement to Pahor, Jansa said "a serious attempt to form a stable government will obviously happen only after all theoretical chances of forming a weak, unstable government fail".
Nevertheless Jansa said he still hopeful of winning enough support in the next few weeks to become prime minister.