Austrian Chancellor Werner Faymann tendered his resignation on Monday following loss of the support of his Social Democratic party colleagues.
He joined the list of prominent casualties of the political upheaval sweeping Europe amid the migration crisis.
Mr. Faymann, 56, a center-left politician who has been chancellor since 2008, said he was giving up his post as head of the Alpine country’s government with immediate effect.
It was still to be determined, he said, who his long-term successor would be.
“This country needs a chancellor whose party is completely behind him,” Mr. Faymann told reporters in Vienna, stating that he was leaving because of insufficient backing within his Social Democratic Party. “This government needs a new start.”
Austrian Vice Chancellor Reinhold Mitterlehner was to be sworn in as interim chancellor later on Monday, a spokeswoman for Austrian President Heinz Fischer said.
Mr. Mitterlehner represents the conservative Austrian People’s Party, the junior coalition partner in the Austrian government.
Mr. Faymann didn’t call for new elections before stepping down, but Austrian media speculated that the government might do so in the coming days.
The next regular parliamentary elections are scheduled to take place in 2018.
Mr. Faymann’s surprise resignation came as Austria’s political establishment continued to reel from a drubbing in the first round of the presidential election in April.
The populist, anti-immigrant Freedom Party recorded its best-ever result in a national election with 35.1% of the vote, more than triple what Austria’s two traditional mainstream parties, the Social Democrats and the center-right Austrian People’s Party, achieved.
“The question is, does one, in these times of great challenges have the strong backing of one’s party?” Mr. Faymann said. “I must answer the question with a ‘No.’”
Mr. Faymann’s resignation is the latest sign of political turmoil across Europe, driven by euro-skeptic, anti-immigrant, populist parties that are gaining traction as mainstream governments struggle to master growing migrant numbers and tackle economic doldrums.
In Denmark, the right-wing populist Danish People’s Party achieved stronger-than-expected election results last year, pushing the centrist government to take tougher measures to keep migrants out.
In Germany, the upstart Alternative for Germany party made landmark gains in state elections in March and now polls at 15%, cementing its status as the most successful right-wing party in Germany since World War II.
Austria has been affected by the influx of migrants because it serves as a key transit point for people bound for Germany and, with a generous social security offering, is a popular destination in its own right.
The country took in roughly 90,000 asylum-seekers last year.
When the influx of migrants Europe spiked last summer, Mr. Faymann initially allied himself with the open-door policy of Chancellor Angela Merkel of neighboring Germany.
But this winter, with the Freedom Party rising in the polls, Mr. Faymann changed course, instituting tough border controls and declaring an upper limit on how many asylum applicants Austria would accept.
The about-face didn’t stop the Freedom Party’s Norbert Hofer from scoring a dramatic victory in last month’s presidential elections—in part, pollsters said, because Mr. Faymann appeared to vindicate the populist party’s position.
The vote sparked turmoil in Mr. Faymann’s party, with some party members questioning the chancellor’s leadership.
Austria, whose population of about 8.5 million people is roughly equal to that of New York City, typically holds limited sway in the European Union. But its political swing to the right has made it a bellwether for the rest of Europe, as a backlash against migrants combines with economic uncertainty to drive voters away from the pro-EU establishment.
Austria’s next political test comes on May 22, in the runoff presidential vote that will pit the Freedom Party’s Mr. Hofer against independent candidate Alexander Van der Bellen, who favors openness toward accepting refugees.