Angela Merkel’s CDU has called for fresh elections in Thuringia after centre-right politicians in the eastern German state used votes from the nativist Alternative für Deutschland to remove its leftwing premier, defying the national party’s refusal to work with the far right.

State elections last October had left the outgoing premier, Bodo Ramelow, of Die Linke, as the candidate with the strongest mandate but without an absolute majority, even in a coalition with the centre-left Social Democrats and the Green party.

Nonetheless, Ramelow was widely expected to be re-elected to form a minority government in Wednesday’s third round of voting.

Instead, the vote threw up a shock result that is likely to reverberate outside the borders of the small state with a population of only 2.1 million, which used to form part of the socialist German Democratic Republic.

Thomas Kemmerich, of the Free Democratic party (FDP), which had gained only five seats in October, beat Ramelow with 45 votes to 44 and immediately accepted his mandate.

The 54-year-old is only the second state premier in German history for the FDP, a pro-business party with liberal roots that was junior coalition partner to the CDU in Merkel’s second term as chancellor. Fourteen months ago, the FDP walked out of federal coalition talks with the CDU and the Greens, citing red lines and compromised principles.

While the ballot in the Thuringian parliament is anonymous, Kemmerich’s victory could have only been made possible with votes from the AfD, whose own candidate in the third round gained zero votes.

The AfD’s branch in Thuringia is dominated by the party’s aggressively nationalist wing. Last September, a court ruled that the AfD’s state leader, Björn Höcke, could legally be termed a fascist, saying such a designation “rests on verifiable fact”.

Speaking in the state parliament in the town of Erfurt amid heckles of “hypocrite” and “charlatan”, Kemmerich insisted “the firewall against the AfD” will prevail”, adding: “I am anti-AfD, anti-Höcke.”

Coming at a time at which traditional conservatives in Merkel’s party are agitating for a more relaxed approach to cooperating with the far right while CDU centrists are eyeing up a future coalition with the Greens, the Thuringia upset could mark a watershed moment.

Alternative für Deutschland’s co-leader Alexander Gauland hailed the outcome as a sign that “excluding the AfD is no longer an option”, while Alexander Mitsch of the CDU’s hardline WerteUnion pressure group said it showed his party “can be successful without the SPD and the Greens”.

Mike Mohring, who heads the Christian Democratic Union in Thuringia, initially insisted the result did not amount to an informal coalition with the far right, arguing that his party “is not responsible for the voting behaviour of other parties”.

The outcome drew wide condemnation not just from Die Linke, whose Thuringia branch is considered one of the most pragmatic within the party that grew out of East Germany’s Socialist Unity party.

Susanne Hennig-Wellsow, the leftwing party’s state leader, threw a bunch of flowers down at Kemmerich’s feet and declined to shake the shock victor’s hands. She told German media that Wednesday’s events had been “long in the making”.

“How far have we come that the FDP allows a state premier Kemmerich to be voted in with votes from the fascist Höcke and the AfD?”, said Die Linke’s co-chair, Bernd Riexinger. “Breaking this taboo will have far-reaching consequences.”

“The events in Thuringia are an inexcusable breach of the firewall,” tweeted the co-leader of the SPD, Norbert Walter-Borjans. “For the ‘liberals’ to become the straw man for the extreme right’s power grab is a scandal of the highest order.”

The FDP leader, Christian Lindner, on Wednesday afternoon rejected accusations the vote had been pre-agreed with the far right, saying he had been surprised by the outcome, and that the AfD’s vote for his candidate had been “purely tactical”.

Lindner insisted Kemmerich was “the candidate of the centre” and appealed to the SPD and the Greens to enter talks with the new state premier. If such a cooperation was ruled out, he said, fresh elections in the state would be necessary.

The leadership of Merkel’s conservatives came out with a clearer, and more critical response. “The FDP has played with fire and set our entire country alight,” said the CDU secretary general, Paul Ziemiak.

He added that a government that had the support of “Nazis like Höcke” could not be the basis for a stable government and urged fresh elections in the state.

The CDU leader, Annegret Kramp-Karrenbauer, said her party’s politicians in Thuringia had acted “against our will”.