Ylva Johansson, the European Commissioner for home affairs, has written a letter to the Polish government asking for "clarifications" about the spiralling cash-for-visa scandal that has gripped the country.
"These allegations are very concerning and give rise to questions regarding the compliance with EU law," said a Commission spokesperson on Wednesday afternoon.
Johansson's letter, which has not been made public, contains a "set of detailed questions" and a request to reply by 3 October.
"We count on the Polish authorities to provide the necessary information to the Commission and to investigate these allegations," the spokesperson said.
Poland's Ministry of Foreign Affairs and its network of consulates are accused of conducting a widespread illegal scheme through which migrants from Africa and Asia paid extraordinary sums of money to obtain a fast-tracked visa.
Since Poland is a member of the passport-free Schengen area, a visa issued by the country grants holders free access to 27 states across Europe, including Switzerland and Iceland.
Germany, which borders Poland, has also asked for an official explanation of the matter.
Reports in Polish media suggest that about 250,000 visas have been issued since 2021 in return for bribes, which cost thousands of dollars each. The trip to Poland was seen as an intermediate step before entering the United States, the desired destination.
In one case revealed by the Onet news portal, a group of Indians paid up to $40,000 for visas and pretended to work on a Bollywood film to fly to Poland and later reach America. Nationals from Hong Kong, Taiwan, Saudi Arabia, Singapore, the Philippines, Qatar and the United Arab Emirates are also believed to have paid inflated fees.
The ruling Law and Justice (PiS) party has admitted the existence of wrongdoing but insisted the scale of the problem is much smaller than what media reports suggest. The government has launched an audit of all consulates abroad, fired several officials inside the Foreign Affairs Ministry and terminated contracts for external services that were handling visa applications.
Piotr Wawrzyk, the deputy foreign minister in charge of consular matters, was dismissed and later hospitalised after reportedly attempting to commit suicide.
Meanwhile, the state prosecutor has charged seven people under suspicion of corruption and participation in the fast-tracked visa scheme.
The scandal has taken the country by surprise and unleashed a media frenzy one month before voters go to the polls in a closely-watched parliamentary election.
The cascade of accusations threatens to blight the public image of Polish Prime Minister Mateusz Morawiecki, who has leaned heavily on his hard-line migration policies as part of his re-election campaign.
Opposition leader Donald Tusk has said the alleged corrupt cell is "probably the biggest scandal of the 21st century in Poland."