Ireland's government will commission a study to find out why voters rejected the European Union's reform treaty in a referendum this month, Prime Minister Brian Cowen said on Wednesday.

Cowen said he saw no obvious ways to end the uncertainty in the 27-member bloc over the treaty, originally meant to put an end to more than a decade of institutional wrangling and bolster the EU's economic and political voice on the world stage.

"We will now press on with the necessary analysis and reflection so that we can understand in some depth what has happened and why," Cowen said.

EU leaders said last week ratification of the Lisbon Treaty by other countries would continue and they would review the way forward together with Ireland at their next meeting in October.

"I made clear our view that it was premature and could be unhelpful to suggest any next steps or timetables at this early juncture," Cowen told deputies in the Dail (lower house of parliament) in an update on last week's summit in Brussels.

EU leaders said ratification, which must take place in all member states, faced a new hurdle as the Czech Republic's constitutional court will probably not rule before October on whether the treaty is in line with its constitution.

French President Nicolas Sarkozy will visit Dublin on July 11 after taking over the rotating EU presidency on July 1.

Ireland's voters almost wrecked EU plans for eastward expansion in 2001 by rejecting the Nice treaty, but the government held a second referendum which approved the pact.

The leader of the opposition Labour party, which backed the Lisbon Treaty, said holding a re-run would not work with voters.

"There can be no question of putting the same package in front of them as before with a request that they might think better of it a second time," Eamon Gilmore said.