Portugal's new centre-right minority government will face an early challenge on November 10 from a left-wing coalition, in the wake of inconclusive elections, parliament said Wednesday.
Prime Minister Pedro Passos Coelho won re-election in the October 4 polls but failed to retain a parliamentary majority, a partial victory that came despite four difficult years of austerity.
His new government will take office on Friday, and he is due to present its programme to parliament on November 9.
The Socialist Party (PS) and fellow parties to its left, which between them won 122 of the 230 seats in parliament, have vowed to pass a motion against the political blueprint in a move that would automatically bring down the government.
Lawmaker Duarte Pacheco, spokesman of the parliamentary group leaders, said the vote would take place on November 10.
Portugal's largest union, the General Confederation of Portuguese Workers which is close to the Communist Party, called for a demonstration outside parliament that day to demand an "alternative politics of the left".
Portuguese President Anibal Cavaco Silva charged Passos Coelho, 51, with forming a government even though his centre-right bloc won only 107 seats.
PS leader Antonio Costa, 54, has claimed he should become premier, saying he could muster a coalition with the Left Bloc -- which is allied to Greece's ruling anti-austerity Syriza party -- along with the Communist and Green parties.
Such a coalition would be unprecedented in the last 40 years of Portuguese democracy and would require the PS to make key concessions on social issues.
However PS parliamentary group leader Carlos Cesar declined to reveal details, saying only: "It will be a transparent accord that will respect the country's commitments."
He vowed: "We will not leave the country without a government. We can propose an alternative way towards a solid government."
Elements of the far left notably oppose the EU budget treaty.
Portugal is the first country to miss the October 15 deadline for sending budgets to the European Commission since a set of new rules for harmonising cross-border economic governance was put in place in 2013.
Luis Montenegro, who heads the parliamentary group of Passos Coelho's Social Democratic Party, said: "The country cannot remain in this uncertainty. The more time goes by the more our international partners have reason to worry."
Spain and Germany have both voiced deep concern about the prospect of a left-wing coalition taking power in Portugal, which is recovering only slowly after emerging from a 78 billion euro ($88 billion) international bailout last year.
Spanish Prime Minister Mariano Rajoy expressed concern over such a possibility, and German Chancellor Angela Merkel said such a coalition would be a "very negative" development.
But two weeks ago, Costa told AFP that a left-wing government would respect trade commitments and avoid any new crises in the eurozone.
"Europe can rest easy, the Socialist Party is not Syriza," Costa said, adding that the Communist Party and Left Bloc "have clearly stated that leaving the euro and renegotiating the debt were not on the negotiating table."
If the new government falls, the conservative Cavaco Silva could ask Costa to form a new government -- though last Thursday he was highly critical of the idea of a government propped up by radical left factions.
Alternatively the president could ask Passos Coelho to stay on as a caretaker prime minister.
Portugal's constitution does not allow another election within six months, so the impasse could stretch well into next year.
Portugal has already seen several minority governments, both left and right, but only one completed a full five-year term -- led by Socialist Antonio Guterres from 1995 to 1999.