Austria's presidential run-off election must be held again, the country's Constitutional Court has ruled.

The decision will give the anti-immigration Freedom Party's narrowly defeated candidate Norbert Hofer another chance to become the first far-right head of state in the European Union.

Mr Hofer lost the May 22 vote to former Greens leader Alexander Van der Bellen by less than one percentage point, with postal ballots having tipped the balance in Van der Bellen's favour.

Witnesses told the court of irregularities in the way the count was carried out, including the processing of postal ballots sooner than they should havebeen.

The court said it was using its strict standard on the application of election rules.