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BERLIN, March 24 (Reuters) - Europe's left-leaning parties appealed on Saturday for the United States to consult further with its allies before going ahead with a controversial plan to site a missile defence shield in Poland and the Czech Republic.

Some European countries fear the U.S. plan to shield itself and possibly other partners from missile threats from Iran and North Korea could raise tensions with Russia, and split Europe into those taking part and those not.

"Please do not make this a unilateral or bilateral decision," Poul Nyrup Rasmussen, president of the Party of European Socialists (PES), said after a meeting of the umbrella group of left-leaning parties across Europe.

"Let us talk about this seriously," the former Danish prime minister told reporters. The PES was meeting ahead of a summit of EU leaders in Berlin to celebrate the 50th anniversary of the European Union.

Kurt Beck, head of the German Social Democrats who are the junior partner in Chancellor Angela Merkel's coalition, suggested there was broad opposition among Europe's left to the U.S. plan.

"With the agreement of our Polish friends...we made it clear we would prefer a solution if possible without a new missile system and certainly without a new arms race," Beck, a fierce critic of the shield, told reporters.

The United States wants to deploy a radar system in the Czech Republic and 10 interceptor missiles in Poland by 2011-12 to counter threats from so-called "rogue states".

It insists it would be technically impossible for such a shield to be deployed against Russian missiles and so cannot be construed as a threat to Moscow, as President Vladimir Putin has contended.

German Chancellor Angela Merkel fears the shield could upset her efforts to launch talks on a new EU-Russia partnership and forge a consensus on re-launching the stalled EU constitution.

Merkel has called for talks on the shield to be conducted within NATO and in full consultation with Russia.

The controversy is due to feature in a European Parliament debate on Thursday with EU foreign policy chief Javier Solana.

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