BERLIN (AFP) -- Spain is prepared to compromise to find an institutional treaty for the European Union but expects Poland to do the same, Prime Minister Jose Luis Rodriguez Zapatero said after talks here on Friday.

"Spain is ready to have a more flexible position so that we can make progress," Zapatero said after a meeting with German Chancellor Angela Merkel ahead of next week's crucial EU summit in Brussels.

Zapatero warned that isolation was not an option for Poland, which is bitterly opposed to proposed changes to the voting mechanism for the 27-country bloc.

"The message that I wish to send to our Polish friends is that we need to look ahead. I am sure that no country wants to stay on the sidelines and that no country will stay on the sidelines," Zapatero said.

He was the first of a host of EU leaders to hold discussions with Merkel over a three-day period as Germany -- which currently holds the EU presidency -- seeks to remove obstacles to its goal of putting the bloc on the road towards a new treaty to replace the rejected draft constitution.

Merkel wants to reach agreement at the summit on a new treaty that would come into force by 2009 at the latest.

Later Friday, the chancellor was to see Dutch Prime Minister Jan Peter Balkenende, whose country helped to wreck the draft constitution when voters rejected it in a referendum in 2005 shortly after the French did the same.

Yet the German chancellor's toughest task was likely to come Saturday when she faces Polish President Lech Kaczynski at a castle outside Berlin in an attempt to answer his country's concerns about a new, simplified treaty.

Poland, which with 38 million inhabitants was the biggest of 10 new EU entrants in 2004, has threatened to veto talks on a new treaty unless it gets its way on the voting system.

It wants the number of votes a country has in decisions affecting the entire bloc to be based on the square root of the country's population and argues that the double majority method that Germany favors would give large countries too much weight in important votes.

Zapatero said in Berlin that his country and Poland must assume their responsibilities. "Poland and Spain have the same number of inhabitants and hold the same weight within the European Union."

Zapatero was also to visit Warsaw Friday, following in the footsteps of French President Nicolas Sarkozy, who said after a lightning visit to Poland Thursday that a compromise deal was possible.

Poland's populist conservative Prime Minister Jaroslaw Kaczynski said he saw some hope of a breakthrough.

"You could say there is a glimmer of hope... but it is still too early to say definitively that this is the case," he told the Dziennik newspaper. But he added: "Let us be clear on this: we are still in favor of the square root system. The arguments that were put to us did not convince us otherwise."

But Polish Foreign Minister Anna Fotyga on Friday accused Germany of putting its national interests above those of other member states.

"It would be good if, during preparations for the European summit, the presidency would make the distinction between its national hat and its institutional, community hat," Fotyga told the Polish parliament.

On Sunday, Merkel's marathon round of diplomacy continues when she meets Czech Prime Minister Mirek Topolanek. Czech President Vaclav Klaus warned this week that his country's concerns about the treaty have not been answered, saying "only cosmetic changes have been made."

Merkel stressed that no one country should be singled out as obstructive.

"There is a whole row of countries who have problems," she said after meeting Zapatero.

The German presidency has given member states a report on the state of play, although it steered clear of the main contentious points over voting. It is proposing "a return to the classical method of treaty change."

The new treaty would therefore be a "reform treaty" amending existing ones rather than an overarching one to replace them, according to the text.

The 18 member states, including Germany, to have ratified the original constitution have made it clear that reducing the new framework to a reform treaty "would represent a major concession," the statement says. However the statement adds that: "All member states recognize that further uncertainty about the treaty reform process would jeopardize the Union's ability to deliver." European Commission President Jose Manuel Barroso warned in an interview with Saturday's edition of French newspaper Le Monde that failure to agree on a new treaty would be a "victory for nationalist forces."