Jarosław Kaczyński, the leader of Poland’s ruling conservative party, is to sue former President Lech Wałęsa over claims that Kaczyński caused the death of his twin brother and 95 others in the Smolensk air disaster.

Lech Kaczyński, then president, died on 10 April 2010 when his plane crashed while trying to land in fog at the Smolensk airport in western Russia.

In 2016 former Solidarity union leader Wałęsa accused Jarosław Kaczyński of being “mentally disturbed” and pressuring his brother to force the plane to land in Smolensk, despite bad weather.

Wałęsa said Jarosław Kaczyński told his brother to make sure the plane landed as scheduled during their last ever phone call, while Lech Kaczyński was in the air.

Recordings of the call have not surfaced but Jarosław Kaczyński said that he spoke to his brother, who called him from the plane, to talk about their ill mother.

“Since they were born [Jarosoław Kaczyński] always made the decisions, Lech went along … he always waited on Jarosław’s decision on everything,” Lech Wałęsa said on Facebook in 2016.

Jarosław Kaczyński has demanded Wałęsa revoke his claims the Law and Justice (PiS) party leader was “sick, mentally disturbed” and that, “during the flight of the plane carrying the Polish delegation to Smolensk, knowing of the unfavourable weather conditions, and driven by heroics, [he] ordered the plane land, which led to the air disaster”.

Wałęsa has not denied saying those things.

“Everything you have said against me … I confirm… I believe that [Jarosław Kaczyński is] responsible for the deaths in Smolensk,” Wałęsa said on Facebook on Wednesday.

Since coming to power in 2015, the PiS government has launched a new investigation into the 2010 presidential plane crash, having long challenged an official report into the crash issued by the previous Polish government.

The earlier report cited a catalogue of errors on the Polish side, while also pointing to errors made by Russian staff at the control tower of Smolensk Military Airport.

A Russian report placed all the blame on the Poles.

The new investigation under PiS claimed the plane was probably destroyed by a mid-air explosion and that Russian air traffic controllers deliberately misled Polish pilots about their location as the plane was approaching the runway.

The 96 Poles on board the plane were heading to commemorations of the 70th anniversary of the Katyń massacres, during which some 22,000 Polish officers were executed by the NKVD, a forerunner of the Soviet Union's secret police organisation the KGB. (vb/pk)