Poland has vetoed the start of talks between the EU and Russia on a new partnership agreement covering energy, trade and human rights.
The move means that it is unlikely the talks can be launched as planned at an EU-Russia summit in two weeks' time.
Poland says Russia must first lift a ban on Polish food imports and ratify a treaty on trade in energy products.
Officials say the EU's credibility will be damaged if a common position is not reached before the 24 November summit.
Russia supplies a quarter of the oil and gas consumed in the EU, and the proportion is set to rise sharply in coming decades.
Polish Prime Minister Jaroslaw Kaczynski said on Sunday that Russia was violating the current EU-Russia co-operation agreement by banning Polish meat, and many other foods.
Polish reservations
"We would like EU member states to show solidarity with Poland regarding Russia," he said, ahead of Monday's meeting of EU foreign ministers in Brussels.
After Poland wielded its veto, External Affairs Commissioner Benita Ferrero-Waldner said she hoped that it would still be possible for the EU to go into the EU-Russia summit with an agreed common position.
However Poland "has not lifted its reservations," she said.
Poland's insistence that Russia should ratify the Energy Charter Treaty - which would help foreign companies invest in Russia's energy market - is no longer shared by all EU countries.
Russia has signed the treaty, and the EU has been trying for years to get Moscow to ratify it, without success.
The European Commission is now aiming to enshrine many of the treaty's principles into the new partnership and co-operation agreement with Russia, once the old one comes to an end next year.
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