BERLIN (Reuters) - German prosecutors have opened preliminary proceedings against Gregor Gysi, one of the Left party's key election campaign figures, over allegations he lied about links with the former East German secret police, the politician said on Sunday.

Gysi, who leads the Left party's parliamentary group, has repeatedly faced allegations in the past two decades that as a lawyer in former East Germany he passed information on clients - some of them known dissidents - to the Stasi secret police.

He said on Sunday he was confident the case would be dropped and saw no reason to reconsider his position in a team of eight that is leading the Left party's campaign for federal elections in September.

"Of course the suit will be discontinued just like previously because I never gave a false affidavit," he said in a post on social media site facebook. "That's why there's not the least reason to reconsider my candidacy."

According to Welt am Sonntag newspaper, the case brought by a former judge concerns an affidavit Gysi gave in 2011 to block the airing of a TV documentary. In the statement he said he had never knowingly or intentionally reported to the Stasi on his clients or anyone else.

Gysi, whose parliamentary immunity was lifted for the proceedings to be opened, has always said he did not cooperate with the Stasi.

Left co-party leader Bernd Riexinger said the claims lacked substance and dismissed them as electioneering.

"Gregor Gysi is our best man. It doesn't surprise us that the others attack him. That's dirty election campaigning," Riexinger told the Berliner Zeitung daily to be published on Monday.

(Reporting by Annika Breidthardt, edited by Richard Meares)