Investigator Frank Taylor's findings challenge official reports by the Russian and Polish authorities which blamed pilot error for the crash in Smolensk, western Russia, in April 2010 which killed 96 people, including then-Polish President Lech Kaczyński, Sky News said.
In a television interview, Taylor told Sky News: "I think they did not do a thorough investigation. It seems to me they came to that conclusion very, very early on and then possibly sought evidence to justify it."
'No doubt there were explosions on board'
Sky News cited Taylor as saying: "What I can say is there is no doubt there were explosions on board before the aircraft hit the ground."
Sky News said that Taylor, who was involved in an investigation of the 1988 Lockerbie air crash in Scotland, had examined high resolution photographs of the Smolensk crash site.
He was cited by Sky News as saying: "If the tip [of the wing of the Polish plane] had been cut off by a birch tree [as a previous official report claimed] the damage wouldn't have looked like this. It would have been bent back - consistently backwards, rather than up and maybe even forward.
"As far as I can see there was an explosion in the wing before the aircraft reached the birch tree.
"Possibly some of the bits hit the birch tree afterwards but the evidence is that this explosion caused the wing tip to come off. Then the aircraft rolled but at a higher altitude than the Russian report suggests,” Taylor was quoted by Sky News as saying.
The son of political activist Anna Walentynowicz, who was on the Polish plane, says he was threatened with jail for speaking out after the wrong body was returned to him and that relatives of crash victims could not be sure they have buried their loved ones, Sky News reported.
Previous report challenged
Poland’s ruling conservatives have long challenged an official report into the causes of the disaster issued by the previous Civic Platform-led government, which 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.
A new commission to probe the crash was set up by Law and Justice, which came to power in Poland in 2015. The party is headed by Jarosław Kaczyński, twin brother of the late President Lech Kaczyński.
The commission said in January that the jet’s left wing was destroyed as a result of an explosion on board.
The commission said that the explosion had “several sources” on the plane.
In April last year, the Polish commission said that the presidential plane was probably destroyed by a mid-air explosion and that Russian air traffic controllers deliberately misled Polish pilots about their location as they neared the runway.