Poland's National Prosecutor's Office is investigating allegations of malpractice involving the issuance of visas by private companies working for the country’s ministry of foreign affairs.
Polish independent media outlets have reported that visa applicants from a number of African and Asian countries were said to have paid fees to intermediaries to speed up the process.
Seven people were arrested last week over alleged irregularities.
The National Prosecutor's Office has said that its investigation involves several hundred visa applications over an 18-month period.
Polish diplomatic offices in Hong Kong, Taiwan, the United Arab Emirates, India, Saudi Arabia, Singapore, the Philippines and Qatar are being investigated.
Former prime minister Donald Tusk, who leads the main centrist Civic Coalition, has called the case "the biggest scandal in 21st century Poland".
Members of the opposition said that as many as 350,000 visas may have been issued incorrectly.
The ruling right-wing Law and Justice party, which has placed its hardline stance on immigration at the centre of its election campaign, has said the opposition’s claims are exaggerated and that the number of irregular cases number in the hundreds.
"There is no scandal. It is not even an affair," said Jaroslaw Kaczynski, the leader of Law and Justice at a campaign rally at the weekend.
Speaking today on Polish radio station RMF, Poland’s minister for foreign affairs Zbigniew Rau called coverage of the case a "cascade of fake news".
State broadcaster TVP, which tows a pro-government line, has offered little coverage to the story.
Referendum questions to deal with immigration issue
The Law and Justice government will hold four referendum questions on the same day as the parliamentary and senate elections on 15 October.
Two of four questions deal with immigration, specifically the government’s opposition to the European Union’s migrant relocation deal, which Poland and Hungary opposed in June.
Earlier this month, the country's deputy minister of foreign affairs Piotr Wawrzyk was dismissed one day after anti-corruption officials conducted a search of the ministry’s offices.
His dismissal has not been directly linked to the cash for visas investigation but deputy interior minister Maciej Wasik said on Friday that Mr Wawrzyk bore "political responsibility for at least a lack of supervision in this matter".
Mr Wawrzrk is a member of Law and Justice and has been removed from the party’s electoral list ahead of next month’s parliamentary election.
The ministry of foreign affairs issued a statement on Friday announcing the dismissal of the director of its legal and compliance department.
The ministry has also terminated its existing contracts with external companies dealing with visa applications, dating back to 2011, and said that audits would be conducted at Polish consulates.
The investigation does not appear to have dented support for Law and Justice.
An IBRiS poll released today shows support for the ruling party at 32.6%, ahead of Mr Tusk’s centrist Civic Coalition on 26.6%.
It marks a 4% fall in support for the main opposition group since the pollster’s previous survey last month.