Warsaw (AFP) - Poland's new right-wing government will on Thursday launch a fresh investigation into what caused a 2010 air crash in Russia that killed then president Lech Kaczynski and 95 others, according to the defence ministry.

The late president was the twin brother of Jaroslaw Kaczynski, leader of the conservative Law and Justice (PiS) party that swept back to power in October after eight years in opposition.

PiS politicians have long insisted the presidential jet crash was not an accident, even though both Polish and Russian investigators found that pilot error, bad weather and poor air traffic control were to blame.

Most of the others who died when the plane came down in Smolensk, western Russia were senior Polish state officials, including its military chief of staff and central banker.

The state delegation was heading for memorial ceremonies in Russia's Katyn forest for thousands of Polish army officers killed by the Soviet secret police in 1940, a massacre the Kremlin denied until 1990.

Defence Minister Antoni Macierewicz, who is the main proponent of the theory that the crash was a political assassination, will ink a document to create a commission of inquiry on Thursday, a ministry statement said.

Fuelling the conspiracy theories is Russia's refusal to return the plane wreckage because of an ongoing probe.

"Six years have gone by since the disaster and we still don't know who's guilty," PiS spokeswoman Beata Mazurek said Wednesday.

"Hopefully the new commission will quickly and effectively determine the causes of the tragedy. Many Poles expect it."

A 2015 opinion poll found that 22 percent of respondents believed the crash was an assassination.

The rest said other factors were to blame, including the 37 percent who believe the pilots were pressured to land despite heavy fog.