Nine former Polish prime ministers and presidents urged voters on Thursday to boycott next month’s planned presidential election, arguing that the ballot, to be held by post, could be unconstitutional and did not guarantee voter confidentiality.

The group included Lech Walesa, who helped overthrow communism as head of the Solidarity trade union movement. Former European Council president and Polish prime minister Donald Tusk and some opposition presidential candidates have already said they would not take part in the May poll.

The ruling Law and Justice (PiS) party has sought to go ahead with the election amidst the ongoing coronavirus pandemic by proposing changes to the electoral code allowing for the vote to take place exclusively by post.

Critics, including human rights groups and election observers, say the legislative changes, which have still yet to be approved by parliament, have been rushed through and could stop the elections from being free or fair.

“The procedure of voting by post in this form and time, as is proposed by the ruling party, are pseudo-elections. We will not take part,” the leaders said in a joint statement.

“The Constitution allows for a state of emergency which would allow for moving the election term while maintaining political stability.”

Opinion polls show fewer than 30% of Poles are likely to cast ballots if the vote is held on May 10 as scheduled.

The head of the Supreme Court, a chamber of which could judge on the validity of the election, ended a six-year term on Thursday, opening the way for PiS to pick a supporter of its contested judiciary overhaul to replace her.

Reuters