The court president, Gerhart Holzinger, announced on Friday that the run-off vote between Hofer of the Freedom party and Green-backed Alexander Van der Bellen would have to be repeated across the whole country after an investigation revealed irregularities in the count of the vote in several constituencies.
The unprecedented ruling comes a week before Van der Bellen was due to be sworn into office.
Hofer had lost out to his rival in a knife-edge election on 22 May, with a majority of only 30,863 votes.
While the Austrian presidency is a largely ceremonial role, the outcome has been seen as hugely symbolic, with the Freedom party seemingly buoyed by growing anti-refugee sentiment and disaffection with the country’s political establishment.
The Freedom party had contested the outcome of the vote after claiming to have detected formal irregularities in 94 out of 117 constituencies, submitting a 150-page formal complaint to the constitutional court.
Over the course of the investigation, it had emerged that several counting centres had begun to process postal votes on the eve of the election, rather than on the day after the election, as Austrian electoral law requires.
Witness statements in court also revealed that election observers in some centres had signed minutes of the vote count without having read them.
While the court emphasised that there was no evidence of the outcome of the election having been actively manipulated, the confirmed irregularities had affected a total of 77,926 votes that could have gone to either Hofer or Van der Bellen – enough, in theory, to change the outcome of the election.
Van der Bellen, a retired economics professor and former leader of Austria’s Green party, was due to be sworn in as president in a week’s time, on Friday 8 July.
Constitutional court president Holzinger said that the ruling “did not turn anyone into a winner or a loser”, but that elections were the “fundamental basis of our democracy” and therefore had to be “fully functional”.
“Even in a stable democracy only the total adherence to electoral standards secures the citizens’ trust in our democracy”, Holzinger said.
The ruling is unprecedented in Austria. In 1970 and 1995, the country’s constitutional court had ordered re-elections in individual councils, but not in the entire country.