Poland is unlikely to back Donald Tusk’s bid to stay on as president of the European council, the head of the country’s governing rightwing Law and Justice party (PiS) has said, in a move that could destabilise the EU during Britain’s Brexit negotiations.

“I don’t imagine the government will back Donald Tusk for a second term,” Jarosław Kaczyński, the former prime minister and Poland’s de facto kingmaker, told the Polska Times newspaper. “Is this the sort of person who should remain at the head of the European council? I have major doubts.”

The two men have long been fierce political adversaries, but their feud has become personal since the 2010 plane crash in Smolensk, Russia that killed 96 people, including Kaczyński’s twin brother, Lech, then Poland’s president.

PiS, which has reopened an inquiry into the disaster, supports a theory that the plane was attacked and claims Tusk – then prime minister – and his government neglected the president’s security and bear, at the very least, moral responsibility for the crash.

Polish and Russian investigators have never found any evidence to support the claim, instead blaming the crash on pilot error in dense fog. Tusk has denied all responsibility, saying he is ready to testify as a witness to the inquiry. But Kaczyński told the paper the EU should be aware that Tusk’s re-election would be “risky” because the investigations “might result in charges” against him.

Tusk was appointed president of the council, which brings together the EU leaders and will define the union’s position in negotiations on Britain’s exit, in December 2014, and was expected to carry on for a second 30-month term after next June.

Although Poland cannot veto Tusk’s re-election, its lack of support would weaken his position.

Diplomats said that with the EU already destabilised by the Brexit vote and rising levels of Euroscepticism, any squabble over the council’s leadership would be damaging.

Some EU leaders, particularly in central and eastern Europe, may be unwilling to vote against Poland. Others, however, might be tempted to back Tusk – generally seen as a popular and effective council president – for precisely the opposite reason.

Poland’s hardline conservative government has won few friends in Brussels. The EU recently launched its first ever investigation into a member state over allegations it had endangered democracy by seeking to strengthen government control over the constitutional court, civil service and Polish public radio and television.

