The EU's executive arm said Tuesday it was still waiting for clear answers from Poland about "extremely worrying" allegations of visa fraud by the country's consulates and contractors.
Polish media reports last month said a system for giving out Schengen visas to people from the Middle East and Africa in exchange for bribes had been put in place through the Polish consulates and some external companies in the countries concerned.
The European Commission asked Poland for explanations in a letter sent on September 19.
But it said a response from Warsaw left key details out.
"We need full clarity to reinstate trust because clarity and trust go hand in hand," commission vice-president Margaritis Schinas told the European Parliament in Strasbourg.
The scandal has put Poland's right-wing governing party under pressure in the run-up to a fiercely contested election this month.
Amid growing tensions within the EU over migration, the affair has heightened fears of a wave of illegal entries into the Schengen free movement zone grouping more than two dozen European countries.
Polish authorities say the scheme may have involved several hundred visas, rejecting claims from the opposition that the real number could be around 250,000.