Poland's parliament has approved the treaty streamlining the enlarged EU's institutions, but President Lech Kaczynski, widely viewed as a Eurosceptic, says he will only sign it into law when Ireland has overcome its voters' opposition to the document.
"I hope all EU member states including Poland will ratify the Lisbon Treaty in the near future," EU Enlargement Commissioner Olli Rehn told a news conference in Warsaw after talks with Polish Foreign Minister Radoslaw Sikorski.
"(This would) free up the European perspective of the western Balkans so that nobody could use the Lisbon Treaty as an excuse to slow down EU enlargement," he added.
Despite his dislike of the treaty, both Kaczynski and Poland's staunchly pro-EU, centre-right government strongly back further EU expansion, especially in the longer term to nearby Ukraine where Europe is vying with Russia for influence.
Some EU countries such as France and Germany are much less keen on further enlargement, especially before the 27-nation bloc has managed to reform its creaking institutions.
All member states must ratify the Lisbon Treaty before it can come into force. The ratification process has been held up by Ireland, whose voters rejected the treaty in a referendum earlier this year.
With Europe's economic outlook darkening fast, Ireland seems unlikely to hold a second referendum on Lisbon soon.
The Czech Republic, which joined the EU with Poland in 2004, has also not yet ratified the treaty. Its prime minister said this week that could happen by the end of this year.
Asked whether he was concerned that the global financial crisis and looming recession might dent already sagging support among EU citizens for more enlargement, Rehn said: "Enlargement is not the problem but is part of the solution."
"New EU member states and the countries of southeast Europe, the candidate countries, have been the fastest growing economies and fastest growing export markets," he added.
EU membership or the prospect of it has underpinned robust economic growth across ex-communist central and eastern Europe in recent years, though some countries -- notably Hungary with its heavy debt load -- are now suffering a sharp slowdown.
Rehn said the current economic turmoil bolstered the case for closer EU ties with Ukraine, whose government is now locked in emergency talks with the International Monetary Fund on financial aid to help weather the crisis.
"This is a further reason why we should rapidly conclude an association agreement with Ukraine ... in these times of economic crisis both Ukraine and the EU benefit from closer economic relations," Rehn told Reuters.
Unlike Croatia and Turkey, Ukraine is not an official candidate for EU membership but last month the Union and Kiev agreed to conclude next year a pact on closer ties known as an association agreement.
Rehn reiterated his view that Croatia could conclude the technical negotiations on joining the EU by the end of 2009 and that Serbia could achieve candidacy status by then.