Poland’s main opposition Law and Justice (PiS) party has split, after former prime minister Mateusz Morawiecki and his allies said they were forced out over factional disagreements on policy direction ahead of next year’s parliamentary election.

Speaking at a press conference, the party’s longtime leader, Jarosław Kaczyński, said dozens of members of parliament effectively left the party after refusing to sign a loyalty oath.

All those who refused to sign the pledge were deemed by Kaczyński to have effectively resigned from the party. The rebel MPs insisted they did not intend to leave, but conceded they may be formally expelled at a party meeting on Tuesday.

“Everyone knew what they were signing, and at the same time everyone knew what the consequences of not signing would be … It’s all done,” Kaczyński said, adding that the decision will be formally processed during the party’s leadership meeting on Tuesday.

Law and Justice is one of Poland’s two most successful political parties since 1989, having ruled the country for 10 years in 2005-2007 and 2015-2023, with its candidates winning four out of eight presidential elections.

Despite losing to a coalition of pro-European parties led by former European Council president Donald Tusk and going into opposition at the last election in 2023, PiS remained the largest party in the Polish parliament – a status it could lose as a result of the split.

The resignations come after months of increasingly bitter disagreements in Poland’s main opposition party as it lost its grip on the Polish right.

Over the past two years, it struggled to respond to the emerging libertarian-right Confederation and the far-right ethno-nationalist Confederation of the Polish Crown, splitting the conservative vote into three.

In April, Morawiecki, who served as Poland’s prime minister between 2017 and 2023, set up a growth-oriented policy group in a bid to broaden the party’s electoral appeal on the centre-right. It is planning to host its inaugural public event later this month.

But the move was harshly criticised by Kaczyński and his allies, who questioned the former prime minister’s loyalty amid rumours of his growing disenchantment with the party’s drift towards the nationalist right and interests of the older electorate.

After tense talks behind closed doors, Kaczyński told all MPs to sign a loyalty pledge committing them not to work with any other political groups or face expulsion, with the deadline set for midnight on Thursday.

Speaking to reporters on Friday, Morawiecki said his group “until the very last moment fought for the unity of the party” as it wanted PiS to follow “a strategy of two lungs” to mobilise younger centre-right voters and entrepreneurs to boost its electoral prospects in 2027.

Morawiecki blamed a circle of “plotters” around Kaczyński for leading the party into the crisis and issuing the loyalty ultimatum. He confirmed the splinter group includes 40 MPs, three members of the European parliament and one senator.

“The situation emerging on the Polish right is that … there is no single dominant force, it comprises many different factions. For me and for us, these disputes within the right are like squabbles within a family, but in fact we will be directing the brunt of our attack at this very bad government by Donald Tusk,” he said.

He declined to speculate on whether the group will form a new party, and instead urged PiS leaders to “take a step back”.

A former senior banker and one-time economic adviser to Tusk, Morawiecki remains one of the most recognisable leaders on the right. But he is also the face of the highly divisive eight years of Law and Justice’s last term in power, seen by critics through the prism of democratic backsliding, conflicts with the European Union, and alleged irregularities and abuses of power.

He is also the leader of the European Conservatives and Reformists Party in the European parliament, a grouping of like-minded parties in Europe, including Giorgia Meloni’s Brothers of Italy.

His departure is likely to further splinter the vote on the right. Hypothetical polls including a new party had it polling at about 6%.

The next parliamentary election is scheduled for October 2027.