 Poland's prime minister vowed on Friday to do his utmost to avoid a referendum in his country on the European Union's reform treaty after being grilled by anxious EU leaders at a summit. Powerful anti-EU nationalists in Poland's opposition Law and Justice party are demanding a referendum, suddenly complicating an expected smooth ratification of the treaty by the bloc's biggest east European member. Speaking after an EU summit, Poland's Donald Tusk said EU leaders had expressed concern at the situation in his country. "All my interlocutors asked me what's going on," he told a news conference. Any referendum in Poland could trigger calls for similar plebiscites in other EU nations, notably Britain, and delay ratification across the bloc. Only Ireland is bound by its constitution to hold a vote. EU leaders hope the treaty, which aims to overhaul the bloc's institutions and improve decision-making, will take effect next January before European Parliament elections in mid-2009. "A referendum is the last resort," Tusk said. He said EU leaders had not expected any problems with Polish ratification since the treaty was negotiated and agreed last year when the current, often Euro-scepetic opposition was in power, and the new government also backed it. But pressure from the Law and Justice party's nationalist wing had pushed its leader Jaroslaw Kaczynski, who as prime minister had signed the treaty last year, to make a U-turn. Kaczynski said his party may support the ratification bill if it included a preamble asserting the supremacy of the Polish constitution over EU law and Poland's right to leave the bloc, something that legal experts have doubts about. Tusk's centrist government has ruled out bowing to those demands, saying it might accept a non-binding resolution on the issue. Parliament is to resume debating the treaty on Tuesday. Tusk said that by obstructing the ratification process, the opposition had tarnished Poland's image abroad. This had weakened his hand in summit negotiations on what burden Poland would have to take under the EU's flagship plan to fight climate change. "Our efforts to rebuild our position in Europe have been partly wasted," he said, appealing to "responsible" opposition parliamentarians to vote in favour of the treaty.