Mr Kaczynski, his wife Maria and 94 other passengers - most of them top state officials - died when their aircraft crashed while trying to land in thick fog in Smolensk on 10 April.
Some victims' families are worried they may not have been given the correct remains for burial by the Russian authorities and are considering exhumation.
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Hide AdAsked whether he planned to exhume the late president's remains, Jaroslaw Kaczynski - who also leads the main opposition party, the conservative-nationalist Law and Justice - said: "I have very strong grounds to believe not all elements which were presented to me belonged to my brother. "
The Polish prosecutors' office, which is investigating into the crash, has denied in the past that there were no doubts about the identity of the remains.