A handwriting analysis has determined that Lech Walesa, the leader of Poland’s anticommunist Solidarity movement and later the president of Poland, was a Communist paid informant in the 1970s, according to a new official report issued on Tuesday.

Mr. Walesa, a Nobel Peace Prize laureate, immediately denounced the report as unreliable and politically motivated, while his supporters insisted that the full picture of his seeming collaboration with the Communist authorities was more complicated than the report suggested.

The report was prepared by the state-run Institute of Forensic Research in Krakow, which analyzed more than 150 documents that were found last year in the home of Gen. Czeslaw Kiszczak, the longtime interior minister during the Communist era, who died in 2015.