VIENNA (AFP, REUTERS) - Austria's highest court on Friday (July 1) annulled May's presidential election result following a legal challenge from the far-right Freedom Party (FPOe), whose candidate lost by a narrow margin, citing irregularities.

"The challenge brought by Freedom Party leader Heinz-Christian Strache against the May 22 election... has been upheld," said Gerhard Holzinger, head of Austria's Constitutional Court.

Norbert Hofer of the FPOe came top in a first round in April but then lost by only 30,863 votes to Alexander Van der Bellen, an independent backed by the Green Party, in a runoff.

Preliminary results on the evening had given Hofer a narrow lead but after some 700,000 postal votes were counted, Van der Bellen was declared the winner of the largely ceremonial post the next day.

A re-run of the presidential run-off election is likely to be held in September or October, former Greens leader Van der Bellen said.

The FPOe, which is topping opinion polls ahead of the next scheduled general elections in 2018 on the back of unease about immigration, launched a legal challenge on June 8 because of "massive irregularities".

These included allegations that tens of thousands of votes were opened earlier than allowed under election rules and that some votes were counted by people not authorised to do so.