Austria's far-right Freedom Party on Wednesday challenged the presidential poll results and announced it seeks a re-election, alleging widespread voting irregularities.
Freedom Party contender Norbert Hofer lost by less than 1 percentage point - 30,836 individual votes - to Green candidate Alexander Van der Bellen in the May 22 run-off vote.
"Without these glitches, irregularities and breaking of laws, Hofer could have become president," party leader Heinz-Christian Strache said after filing an official challenge at the Constitutional Court.
The lawyers of the anti-immigration Freedom Party said they found evidence that more than 573,000 absentee and postal ballots had been handled without the presence of local election commissions, creating the risk of tampering.
"One doesn't have to be a conspiracy theorist to have a bad feeling about this," Strache told reporters.
Robert Stein, the Interior Ministry's chief election officer, told dpa that it is not illegal to pre-sort absentee ballots as long as the seals of the envelopes are not broken. He did not react to Strache's allegation that thousands of such ballots had also been prematurely opened.
In Austrian elections, left-wing and green voters traditionally account for a disproportionate share of absentee and postal ballots.
Strache demanded that citizens should no longer be allowed to send in their voting sheets by mail, arguing that this system could not guarantee a secret and individual voting procedure.
"It needs to be abolished," he said.
The Constitutional Court aims to rule on the matter by July 8, when Van der Bellen is set succeed Social Democratic incumbent Heinz Fischer, a court spokesman said.
The court can order a new election in affected voting districts or the entire country if it finds that there were unlawful practices and that these affected the overall outcome.
"I think a re-election is very realistic," Strache said.
The Freedom Party has questioned the legality of Van der Bellen's win ever since the result was announced on May 23 and has suggested that established mainstream parties have rigged the system.
"Challenging an election is an important democratic tool," leading Green parliamentarian Dieter Brosz said. "But the Freedom Party's statements over the past days show that this instrument is being used for tactical party politics in this case."
Van der Bellen's campaign manager Lothar Lockl said the Freedom Party had the right to take these legal steps.
"Now it's up to the Constitutional Court to decide. We are relaxed about this," Lockl said.