Four people have been charged in an investigation into suspected bribery by a Gulf state, which Belgian media have alleged is Qatar.
Around €600,000 (£515,000) has been seized by Belgian police from members of the European Parliament in what is suspected to be slush money from the World Cup host.
Prosecutors in the case believe the Gulf state is guilty of attempting to influence parliament decisions with gifts or money.
Michiel van Hulten, a former MEP and director of the anti-corruption organisation Transparency International EU said "While this may be the most egregious case of alleged corruption the European Parliament has seen in many years, it is not an isolated incident."
EU's foreign policy chief Josep Borrell added, "Certainly the news is very, very worrisome."
"We are facing some events, some facts, that certainly worries me as a former president of the European Parliament."
In total, six people were detained for questioning following raids on MEP households, with two eventually released.
Greek MEP and European vice president Eve Kaili is among those arrested.
EU President Roberta Metsola flew to Brussels to witness the police raids, as required by the Belgian constitution.
A spokesperson for Metsola said she had “decided to suspend with immediate effect all powers, duties and tasks that were delegated to Eva Kaili.”
Kaili has previously been openly supportive of Qatar, calling the Gulf state a “frontrunner in labour rights.”
Qatar won the right to host this year’s FIFA World Cup in 2010 and the 12 years that have followed have been clouded in controversy.
The country has been criticised for its attitudes towards homosexuality and its treatment of migrant workers, with some figures suggesting upwards of 15,000 migrant workers had died working to help Qatar prepare for the World Cup something it has consistently refuted.
Concerns about the way the current World Cup was awarded to Qatar were further fuelled by an inquiry that found nearly all the 22 members of football governing body FIFA’s executive committee had accepted bribes at some point.
In 2020, the US Department of Justice explicitly accused Russia, which hosted the 2018 World Cup, and Qatar of using bribery to rig the votes for both tournaments.