Polish investigators allege Solidarity leader and former president had agreed to be paid for information by Soviet-era intelligence

Lech Wałęsa, the leader of Poland’s Solidarity movement in the 1980s and the country’s first president of the post-communist era, has pledged to defend himself in court against a new claim that he was a paid informant for the country’s Soviet-era secret service.

The state-run National Remembrance Institute (NRI) said on Thursday that documents seized from the home of the last communist interior minister, the late General Czesław Kiszczak, included a letter signed with Wałęsa’s code name, “Bolek”, in which he committed to providing the intelligence services with information.

Receipts for money signed by the former trade union leader had also been found, said the institute. The head of the NRI, Łukasz Kamiński, said the 279 pages of documents “seem to be authentic” and would be made public.

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The 72-year-old Nobel peace prize laureate, currently travelling in Venezuela, has denied the allegations, insisting in his micro blog: “There can exist no documents coming from me. I will prove that in court.”

His son Jarosław Wałęsa, an MEP, told the state-run news agency PAP: “The documents have zero value because we all know that the documents, especially those that are about my father, have been manipulated or forged.”

Poland’s nationalist Law and Justice government, which came to power in November, has pledged to purge the country’s political elite of communist collaborators. Its deputy prime minister, Mateusz Morawieski, has said a “cleanup” like that carried out in Germany after the fall of the German Democratic Republic is necessary.

Wałęsa has proved a thorn in the side of the new Polish government. A few weeks after Law and Justice came to power, he called for early elections to head off what he called “a threat to democracy” posed by its government.

The Law and Justice party leader, Jarosław Kaczyński, is a former senior official of the Solidarity trade union movement that Wałęsa co-founded and which is emblematic of Poland’s emergence from communist domination. But the two men fell out years ago.

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Their conflict goes back to 1990 when Wałęsa – soon after being elected to a five-year term as president of Poland – dismissed Kaczyński and his late twin brother, Lech, from positions in his office. Kaczyński has since maintained that Wałęsa was a collaborator with the communist secret police, the Służba Bezpieczeństwa.

However, in 2000, the Warsaw appellate court declared Wałęsa’s so-called “lustration statement”, in which he insisted he was innocent of the charge of collaboration, to be true.

The accusations of collaboration resurfaced this week after Kiszczak’s widow reportedly offered to sell the documents concerning Wałęsa to the NRI for 90,000 zlotys (£15,800). But prosecutors seized the documents because the law requires important historic papers to be handed in.

One document reportedly suggests Wałęsa had been de-registered as an informant as late as 1976. Five more files of paperwork had yet to be opened, said Kamiński.