A Polish appeals court upheld a lower court's verdict in a slander case and ruled Monday that pro-democracy fighter and former president Lech Walesa must apologize to the leader of the country's right-wing ruling party.

Law and Justice party leader Jaroslaw Kaczynski, Poland's most powerful politician, sued Walesa for blaming him on social media for the 2010 plane crash in Smolensk, Russia that killed President Lech Kaczynski, Kaczynski's twin brother, and 95 others.

The Gdansk Provincial Court in December ruled Walesa had made "heavy accusations" without evidence to back them up and ordered him to apologize to Kaczynski on the radio, in a personal letter and other formats. The provincial court rejected a demand for Walesa to pay 30,000 zlotys ($8,000) to a charity.

Both men appealed. Walesa said he also would appeal Monday's decision to Poland's Supreme Court and "if need be," to the European Court of Human Rights in Strasbourg, France.

In one of the disputed posts written during 2015-2017, Walesa alleged that Kaczynski was "guided by bravado" and during a phone conversation with his traveling brother pushed for the plane to land at the poorly equipped Smolensk airport despite heavy fog.

Kaczynski blames the crash on Poland's government at the time, which was led by another foe, current European Council president Donald Tusk, alleging it neglected the president's security.