Jaroslow Gowin, leader of the Accord party, was dismissed as deputy prime minister by Prime Minister Mateusz Morawiecki.
One of Poland's three ruling parties announced on Tuesday it was leaving the conservative-nationalist coalition after its leader was dismissed by the prime minister.
Jaroslow Gowin, leader of the Accord party, a junior partner in the United Right coalition helmed by the Law and Justice (PiS) party, was dismissed as deputy prime minister by Prime Minister Mateusz Morawiecki.
"We are leaving the government with our heads held high. My resignation means the end of the United Right and the break-up of the coalition," Gowin told reporters on Tuesday evening.
Accord has 10 lawmakers in the lower house of parliament and Gowin's resignation could mean the loss of the government's majority.
Until now, the United Right coalition had 232 of the 460 seats in the lower house.
"I am not convinced that we will lose the majority because of this," government spokesman Piotr Mueller told reporters. "I am convinced that there will be people in the United Right and in the rest of the Polish parliament who will support the beneficial reforms we have proposed."
Gowin said he learned of his dismissal through the media, when government Mueller announced it at a press conference.
The two parties are sparring over the Polish Deal economic programme with Mueller criticising Gowin for not having made enough progress on the reform.
Gowin defended himself, saying the programme is largely tax-based and would primarily impact the middle class.
"We announced that the United Right would not raise taxes. However, the tax bill recently presented by Prime Minister Mateusz Morawiecki means a drastic tax increase not only for entrepreneurs, but for all enterprising people, the middle class," he told reporters.
To maintain its majority, the PiS will now have to rely on MPs from other parties, especially from the far-right.