WARSAW (Reuters) - Poland promised on Monday to put up a tough fight at the European Union’s anniversary summit later this week, with the country’s most powerful politician saying Warsaw will not bow to any form of a multi-speed Europe.

The March 25 EU summit marking the 60th anniversary of the Treaty of Rome, which paved the way for European integration, will ponder the bloc’s future without Britain.

Some euro zone leaders back the idea of letting some member states push ahead with further integration without the whole bloc following suit.

EU chief executive Jean-Claude Juncker, who presented options for reforming the bloc to be discussed at the Rome summit, also has spoken positively of some states pushing ahead more quickly with integration.

“We cannot accept any announcements of a two-speed Europe,” Jaroslaw Kaczynski, the leader of the governing euroskeptic Law and Justice Party (PiS), told the weekly wSieci.

“This would mean either pushing us out of the European Union or downgrading us to an inferior category of members,” he said. “We must oppose that with all firmness.”

Poland was the sole opponent to the reappointment on March 9 of Donald Tusk, Kaczynski’s political nemesis, as the European Council chairman. That a move that further isolated Warsaw from Brussels and cost PiS some domestic support.

A new poll by the IBRiS pollster for the Rzeczpospolita daily showed that support for PiS was down 5 percentage points after Warsaw’s fight over Tusk, while the main opposition party that once had Tusk as its chief gained 10 points.

Kaczynski did not reveal any strategy or concrete steps Poland will present at the Rome summit, saying only: “We will fight for Polish interests with full determination.”

The European Union is effectively already multi-speed through the enhanced cooperation mechanism that lets groups of countries cooperate on specific issues without engaging the whole bloc. The euro zone is the clearest example of this.

Prime Minister Beata Szydlo will head the Polish delegation.