Austria's far-right Freedom Party has challenged the result of last month's election result, the constitutional court have announced.
Former Greens leader Alexander Van der Bellen beat Freedom Party's Norbert Hofer in May's presidential election after postal ballots were counted.
The margin of Van der Bellen's election victory was just 31,000 votes - less than one percentage point.
Alexander Van der Bellen of the Greens has beaten right-wing Norbert Hofer of the Freedom Party to become Austria's President.
Hofer conceded defeat on his Facebook page as officials announced that Van der Bellen won by 31,026 votes.
Thanking his supporters, Hofer said: "I will remain loyal to you and my contribution for a positive future in Austria. Please don't be despondent."
Austrians went to the polls on Sunday in a presidential contest that had migration as a key issue, highlighting how the refugee crisis is shaping the continent's politics.
- ITV News Report by Europe Editor James Mates
The two Austrian candidates vying for presidency have split the country exactly in half, according to voting projections.
Most of the votes have been counted, and the projection made by the state broadcaster has the two candidates at 50% and 50% in what has been called a statistical dead heat.
Postal votes are yet to be counted, however, and the new president is now expected to be announced on Monday.
Austria's Ministry of the Interior has announced that the preliminary result of the presidential election shows Norbert Hofer on 51.9% of the vote, ahead of Alexander Van Der Bellen on 48.1%.
Norbert Hofer is the candidate for far-right party the Freedom Party, while Van Der Bellen is an Independent candidate.
The full result will not be known until Monday as postal votes need to be counted.
The result of Austria's presidential election will not be known until Monday, said far-right Freedom Party candidate Norbert Hofer.
"We will have to wait until tomorrow," Hofer told broadcaster ORF.
Nearly all votes that have been cast in polling stations have been counted, with two candidates in a dead heat.
Postal ballots, which will not be counted until Monday, will determine the winner of what has been the closest race in living memory.
The far-right Austrian candidate is marginally ahead in presidential vote, exit polls suggest.
The race is too close to call with Norbert Hofer on 50.2% and former Greens leader Alexander Van der Bellen on 49.8%, according to a projection by Sora, with a margin of error of two percentage points.
Migration was a key issue in the presidential campaign, highlighting how the refugee crisis is shaping the continent's politics.
Europe Editor James Mates tweets:
First results show too close to call in Austria election. Hofer 50.7% van der Belen 49.3%. 57% votes counted
Many votes have been counted already in #Austria election. Importantly none counted at all in Vienna. Probably no result now till tomorrow
A gunman has killed two people and wounded 11 more before shooting himself dead at a concert in western Austria.
BREAKING: Police say gunman kills 2, wounds 11 at concert in western Austria, then kills himself.
Police said the gunman fired shots into a crowd of about 150 people attending an open air concert in the early hours of Sunday morning near Nenzing, a town in Voralberg province about 24 miles east of Austria's border to Liechtenstein.
A loud argument between the gunman and a woman in a nearby parking lot was heard moments before the shooting, police added.
State broadcaster ORF said it occurred on property used by a motorcycle club, and the man apparently used a rifle or similar long weapon.
Surviving victims are understood to be in a serious condition.
Austrians go to the polls on Sunday where voters could elect the European Union's first far-right head of state.Read the full story ›