Although they did not give a reason for their departure, Polish media pointed the finger at a conflict the duo had with the defence minister.

"General Rajmund Andrzejczak submitted his resignation ... on Monday," Andrzejczak's spokeswoman, Colonel Joanna Klejszmit, told AFP.

"Like any soldier, he's entitled to resign without giving a reason," she said.

The army head of operations, General Tomasz Piotrowski, also stepped down.

Polish media said the two officials, who had been in post since 2018, decided to leave after an extended conflict with Defence Minister Mariusz Blaszczak, and were opposed to attempts to involve the army in the campaign for Sunday's closely fought legislative elections.

The opposition has now demanded the resignation of the defence minister as well.

"There's a war in Ukraine, a war in the Middle East and the Polish army is falling apart," Krzysztof Gawkowski, head of the parliamentary group of the opposition New Left party, posted on X, formerly Twitter.

"The actions by the (ruling) Law and Justice party have left us defenceless during the worst possible crisis," he said.

Poland's relations with neighbouring Ukraine have become brittle even though Warsaw has up to now been a leading supporter of Kyiv following Russia's invasion on February 24, 2022.

While the populist Law and Justice party is set to win most votes in the parliamentary elections, it appears set to fall short of a majority.

The most obvious partner is the far-right Confederation party, which wants Poland to stop sending aid to Ukraine and has criticised the rights of Ukrainian refugees.