VIENNA (Reuters) - Austria's far-right Freedom Party said on Wednesday it is suing one of the judges who ordered a re-run of the country's presidential election for saying the party started preparing its challenge against the result before the vote took place.

Johannes Schnizer sits on the 14-member Constitutional Court, which found widespread infringements of rules on how postal ballots were counted in the May presidential run-off that Freedom Party candidate Norbert Hofer narrowly lost.

In examining the Freedom Party's challenge in June, the court found no proof of foul play in the ballot count but several cases of election officials cutting corners to ensure votes were processed quickly. The re-run will be held on Dec. 4.

Last month, Schnizer said in interviews with Austrian media that Freedom Party election observers knew their party would challenge the outcome and did not tell election authorities of any infringements they should correct.

"It is a grave accusation. It is false," said Freedom Party lawyer Michael Rami, adding that he was preparing a civil case for slander and a criminal defamation case. The maximum prison sentence on the criminal charge is one year, he said.

The Freedom Party (FPO) issued a statement saying it felt it had no choice but to take legal action.

"We cannot let the accusations made publicly by Mr Schnizer against the FPO and Norbert Hofer stand," it said.

Schnizer's lawyer Michael Pilz said: "I cannot understand why the FPO is reacting in such a sensitive way. If the FPO is of the opinion that it must settle this in court, we will deal with it."

(Reporting by Francois Murphy and Kirsti Knolle Editing by Jeremy Gaunt)