WARSAW (Reuters) - Poland’s government-affiliated history institute said on Tuesday it is investigating allegations of perjury by Lech Walesa, the Nobel Peace Prize-winning leader of the Solidarity movement that helped bring down communist rule.

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It was the latest twist in a long-running feud between Walesa and the ruling conservative Law and Justice party of Jaroslaw Kaczynski, who is also a former anti-communist activist who fell out with Walesa in the 1990s.

An investigator for the Institute of National Remembrance (IPN) said the case concerns sworn testimony filed by Walesa that said documents suggesting he been a paid informant for the communist secret police in the 1970s were forged.

Walesa, also a former president, has acknowledged once signing a commitment to inform but insists he never fulfilled it, and a special court exonerated him in 2000.

In January, IPN experts said that handwriting experts proved the authenticity of documents suggesting that Walesa had collaborated with communist rulers.

The IPN investigation, which began in late June and was ordered by the Public Prosecutor General, follows notifications of a suspicion of a crime sent by two private people, according to Robert Janicki, an IPN investigator.

“At the moment analysis of cases in which Lech Walesa filed a testimony related to the documents is being conducted,” Janicki told Reuters. “The case is in a preliminary stage.”

Adam Dominski, head of Walesa’s office, told the state-run PAP news agency on Tuesday there is no evidence to substantiate the documents. “The president maintains that the documents are not genuine and not authored by him,” Dominski said.

It was unclear why the news of the investigation became public only on Tuesday, two months after it began.

Janicki said that at the time of Walesa’s testimony, the penalty for false testimony was up to three years in prison.

Under Polish law, the IPN can lead investigations into historical acts committed against persons of Polish nationality and cases related to the destruction, concealment, removal and alteration of documents subject to transfer to the IPN.