Poland’s supreme audit office has accused prime minister Mateusz Morawiecki of exceeding his powers, as it unveiled a highly critical report into the government’s attempt to hold last year’s presidential election by post because of the pandemic.

The salvo by the supreme audit office (NIK) is the latest in a series of disputes over last year’s election, which was meant to be held in May, but was eventually postponed until June as coronavirus swept through Europe.

It is also the latest in a series of clashes between the ruling Law and Justice party and Marian Banaś, a former finance minister who was put in charge of the NIK in 2019 thanks to the support of politicians from the ruling camp, but has since become a thorn in the government’s side.

Representatives of NIK, which is responsible for auditing government spending, on Thursday said the attempt to hold the presidential election by post in May — which was ultimately abandoned after disagreements in the ruling camp — had cost at least 76m zloty ($20.2m).

They also said that there had been no legal basis for the prime minister to give any orders to two state-controlled entities, the Polish Post and the Polish Security Printing Works (PWPW), in relation to holding the election, such as the printing of voting cards.

“The only body entitled to organise elections was the State Election Commission,” Banaś said during a press conference. “Organising the elections on the basis of an administrative decision should not have happened and was without legal basis.”

He said the NIK had informed prosecutors of possible crimes committed by the boards of the Polish Post and PWPW, which were involved in the preparations for the postal ballot.

The Polish Post said “categorically” that “all its actions taken to implement the prime minister’s decision of April 16 2020 were founded on legal provisions”. PWPW said it considered NIK’s move “unjustified” and “baseless”.

Banaś added that the NIK was analysing whether to notify prosecutors of concerns relating to the actions of other parties involved in the preparations for the election.

The government said that “all decisions on beginning technical preparations for postal voting in the presidential elections were in accordance with the law”.

“All the actions [of the prime minister and the head of the chancellery of the prime minister] were aimed at holding elections by the constitutional deadline,” the government’s information office said in a statement.

“The prime minister never called for presidential elections or for postal voting. The goal of the actions taken was to allow the participation in the elections of those who were entitled to vote, but whose life and health were at risk as a result of the pandemic.”

Jacek Sasin, minister for state assets, took a similar line, and told Polish state radio that the NIK report was “a certain element in the fight between the government and . . . Marian Banaś”.

Banaś has been under pressure to step down from his post since media reports emerged alleging that a building he owns was used as a brothel. In an interview with Politico, he dismissed the allegations as a “smear campaign” aimed at ousting him.

He concluded his press conference by drawing attention to the fact that the NIK was one of a series of institutions targeted by fake bomb threats earlier this week, and to an email sent to the NIK this morning falsely claiming that Banaś’s son was going to commit suicide.

“I ask you yourselves for a comment on this,” he said to the assembled journalists.