The two generals did not give a reason for their departure.

But Polish media said they had decided to step down after a long-running conflict with Defence Minister Mariusz Blaszczak, and were opposed to attempts by Blaszczak to involve the army in the campaign for Sunday's closely fought legislative vote.

"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 quit.

Poland's National Security Bureau (BBN) said President Andrzej Duda would accept the departure of Andrzejczak and Piotrowski, who had been in post since 2018.

"From the president's point of view, from the BBN's point of view, it is absolutely crucial to maintain continuity of command and efficiency of operations," BBN head Jacek Siewiera told reporters.

He added that nominations for candidates to fill the two vacant posts would start on Tuesday.

Defence minister Blaszczak accused Piotrowski earlier this year of not properly searching for debris from a stray missile that fell in northwest Poland, and of failing to inform anyone of the incident.

The remains of a Russian KH-55 cruise missile capable of carrying nuclear warheads were found by a passer-by near the northern city of Bydgoszczin in April, around four months after it landed, Polish media said.

Bydgoszczin is around 500 kilometres (300 miles) west of NATO member Poland's border with Belarus.

- 'Army falling apart' -

Prime Minister Mateusz Morawiecki said that he had only been informed of the incident at the end of April, even though Andrzejczak said he had told those in charge "at the time of the event".

Donald Tusk, the leader of the largest party in opposition, said he had been told that 10 other senior army officers had also resigned, a claim which has been denied by the general staff.

The opposition is now calling for the defence minister to step down 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 strained even though Warsaw has up to now been a leading supporter of Kyiv following Russia's invasion on February 24, 2022.

Poland has taken in over one million Ukrainian refugees, sent large amounts of weapons and aid to Kyiv and acted as a key transit country for Western supplies.

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.