By Lidia Kelly

WARSAW (Reuters) - Poland's government said it will do everything possible to postpone an expected decision by European Union leaders to reappoint Polish former prime minister Donald Tusk on Thursday as chairman of their summits.

Diplomats have said European Union leaders, who will meet in Brussels, are virtually certain to extend Tusk's mandate as head of the council, which groups the EU's 28 national leaders, by a further 30 months.

Poland's ruling right-wing Law and Justice party (PiS) accuses Tusk, a soft-spoken centrist, of overstepping his mandate by, for example, criticizing the Warsaw government during its standoff with the opposition over media freedom.

Tusk and PiS leader Jaroslaw Kaczynski, the country's most powerful politician, are also long-standing bitter rivals in Polish politics.

"We will do everything to ensure that the reappointment does not take place today," Foreign Minister Witold Waszczykowski told the private TVN24 news channel.

"We will inform our (EU) partners that today's summit will be threatened if they push their vote today," without elaborating further on Warsaw's position.

A source in the Polish government said that Poland will first insist on delaying the reappointment and then seek a special summit on choosing the next European Council president after Tusk's term ends at the end of May.

Last week the Polish government proposed another Pole, a member of the European Parliament, to replace Tusk.

German Foreign Minister Sigmar Gabriel said after meeting Waszczykowski on Wednesday in Warsaw that the priority was to maintain unity.

"One can only hope that the conflict will be resolved so we don't have a great divide in Europe," Gabriel said, when asked about Poland's stance on Tusk.

Prime Minister Beata Szydlo, loyal to Kaczynski, will represent Poland in Brussels.

(Additiona reporting by Sabine Siebold; Writing by Lidia Kelly; Editing by Dominic Evans)