European Union leaders sought Tuesday to downplay a simmering row over whether Europe could split into a two-speed bloc if sputtering talks on a first-ever constitution definitively fall apart.

Irish Prime Minister Bertie Ahern, who took over the EU's presidency this month, said the dispute -- which erupted after constitutional talks collapsed last month -- was a question of emphasis rather than substance.

European Commission chief Romano Prodi, in Dublin to be briefed by the Irish presidency on its programme for the next six months, also affirmed there was no fundamental discord.

But Prodi reiterated his view that, if the expanding union does not agree on a new constitution by the end of this year, some countries will have to move ahead.

"If we are unable to do that we can't wait forever. We can't stop Europe. There will come a moment when somebody must give the example and go on," he told reporters at a joint press conference with Ahern.

"It's not a case of fighting... I think I am perfectly in line with the Irish prime minister. We have a different role but we are working in the same direction," he added.

Ahern repeatedly refused to comment on Prodi's forecast, which echoes comments notably by French and German leaders after the breakdown of last month's talks in Brussels.

"I totally understand that," Ahern said, referring to Prodi's comments. "But from our perspective we want to move together, we want to do that this year and we want to get on with it now."

The 15-member European Union had hoped to agree a historic constitution in time for its biggest-ever expansion on May 1, when 10 new mostly ex-communist countries will join the bloc.

But an EU summit in mid-December collapsed amid a row over voting rights in the enlarged bloc, with Spain and Poland notably refusing to surrender generous rights secured at a summit in Nice three years ago.

French President Jacques Chirac immediately revived his long-cherished idea of "pioneer groups" of countries, forging ahead in different policy areas. German Chancellor Gerhard Schroeder also evoked the possibility.

But many other countries, fearing being left in Europe's slow lane, are equally firmly against the idea.

"If everyone sticks to their positions, we will have a two-speed Europe, and that's a disastrous idea for the European Union," Polish President Aleksander Kwasniewski warned only hours after the failure at the Brussels summit.

Ireland, which took over the EU reins from Italy on January 1, has promised to report back to an EU summit in March on prospects for resuming the negotiations.

But many EU officials are forecasting that any progress on the issue may have to wait until the Dutch EU presidency in the second half of the year, and possibly beyond.

Ahern, faced with repeated questions about the two-speed option, underlined that he wanted an EU constitution "as soon as possible," but offered no predictions on what that might be.

"What we want to do is to look at this positively. We don't want to look at... what might happen in a few years' time," said the Irish leader.