Lepper is the leader of the Self Defense party, a junior coalition member with 46 seats in the 460-seat parliament.
Soon after being fired, Lepper withdrew his party from the coalition, raising the possibility a general election will have to be called two years ahead of schedule, a BBC correspondent reported.
"The aggression of the opposition makes it impossible to have a minority government", Kaczynski told a Warsaw news conference.
Without Lepper's party, Kaczynski can only count on 203 parliamentary votes, short of the 231 required for a majority.
The opposition Democratic Left Alliance later said in a statement it would seek a no-confidence vote to force an early election, the report said.