Polish President Lech Kaczynski signed the European Union's reform treaty into law today, leaving the Czech Republic as the only country still to ratify the document.
Kaczynski, a eurosceptical conservative, signed the Lisbon Treaty at a ceremony in the presidential palace attended by the heads of the European Commission and the European Parliament.
Before signing, Kaczynski stressed that the EU remained a union of sovereign nation states and said it must remain open to new members, including countries in the Balkans and Georgia.
"The EU remains a union of nation states, a strict union, and let it remain so... Within a union of sovereign states we will achieve increasing successes," Kaczynski said.
"We now have 27 member states. I am deeply convinced this is not the end... The EU, a successful experiment without precedent in human history, cannot be closed to those who wish to join... not only in the Balkans but also countries like Georgia."
The president of the European Commission, Jose Manuel Barroso, the head of the European Parliament, Jerzy Buzek, who is a former Polish prime minister, and Sweden's Prime Minister Fredrik Reinfeldt were among those attending the ceremony.
Sweden holds the EU's rotating presidency.
The Lisbon Treaty is designed to give the 27-nation bloc a long-term president and a stronger foreign policy chief. It can only take effect when all member states have approved it.
Czech President Vaclav Klaus set out his terms on Friday for signing the treaty, demanding an exemption to protect Prague from post-war property claims and safeguard the sovereignty of the judiciary.
Poland and Britain have already won opt-outs on the application of some of the provisions of a Charter of Fundamental Rights which will be given binding force when the Lisbon Treaty is ratified.