The head of Belgium's Flemish Christian Democrats, Yves Leterme, has abandoned efforts to form a coalition government.
Mr Leterme informed King Albert II that he had been unable to come up with a plan acceptable to both Dutch- and French-speaking politicians.
French-speaking politicians in Wallonia have rejected his demands for greater autonomy for the country's regions.
With no obvious coalition alternative, it is unclear what the king's next move will be, correspondents say.
Prime Minister Guy Verhofstadt has continued to run the country despite losing the 10 June election.
Mr Leterme was nominated to form a government after his party came ahead in the poll, winning 30 seats in the 150-seat lower house of parliament, ending eight years in opposition.
A spokesman for the Christian Democrats said Mr Leterme had decided to resign as coalition negotiator after a deadline he set for all four parties to agree to his proposals passed at midday on Saturday.
Mr Leterme informed King Albert of his resignation - which was accepted. He is due to make a short statement to parliament, the spokesman said.
His party said the Flemish and French-speaking Liberals had signed up to his reform programme, but that the French-speaking Christian Democrats had declined to do so.
Correspondents say the French-speaking southern region of Wallonia fears that greater regional self-rule will deprive it of federal tax revenues and have a negative impact on social services.
Unemployment in Wallonia is more than double that of Flanders, and twice as many Walloons as Dutch-speaking Flemings are employed by the state.
The Walloons make up about 40% of Belgium's 10.5m population, while the Flemings, who are based in the northern half of the country, represent the majority.
No single party bridges the linguistic and geographic gulf between Belgium's two regions.