Three diplomats told AFP they expected eastern EU members Poland, Hungary and the Czech Republic to face sanctions over their refusal to participate in relocating 160,000 refugees under a 2015 plan to ease the burden on frontline states Italy and Greece.
The European Commission headed by Jean-Claude Juncker is expected to make a decision on legal action today, with a formal announcement tomorrow.
But Poland's Foreign Minister Witold Waszczykowski responded: "We're not at risk.
"We won't react, we're not going to explain to them (the Commission) that this is illegal."
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"Most of these people aren't refugees, they're migrants, who arrived in Europe illegally and they don't want to be re- settled in Poland, this would have to be done by force," Waszczykowski added.
Brussels set a June deadline for Warsaw and Budapest to start accepting mainly Syrian, Eritrean and Iraq asylum seekers. Prague has also come under pressure after effectively dropping out of the relocation plan.
Under "infringement" proceedings the European Commission, the 28-nation EU's executive arm, sends a letter to national governments demanding legal explanations over certain issues, before possibly referring them to the European Court of Justice.
Rafal Bochenek, spokesman for Poland's right-wing government, dubbed the Commission's threats of legal action "surprising", accusing it of "weakness" and "inability to cope" with migrant crisis.
Warsaw has offered to send humanitarian aid to refugees camps or war zones in the Middle East.
Echoing earlier fiery rhetoric by Prime Minister Beata Szydlo, Bochenek said that the refugee relocation plan posed a security "threat" to EU members.
By the start of June, less than 20,000 of 160,000 refugees had been relocated under the plan, created in response to Europe's biggest ever migration crisis.
They said some countries were setting unacceptable conditions by refusing Muslims, black people or large families, with Eastern European states the worst for discriminating on religious or racial grounds.