The politician, a member of the Panhellenic Socialist Movement (PASOK), has been expelled from her centre-left party in Greece, and suspended by Socialists and Democrats Group in the European Parliament.

AFP has reported that Kaili is one of five arrested following an investigation by Belgian prosecutors into criminal organisation, corruption and money laundering.

Around €600,000 (£516,000/$632,000) in cash was seized on Friday (December 9) when police in Brussels searched 16 addresses, and computers and mobile phones were also taken.

Belgian newspaper L’Echo has reported that several bags of money were found at Kaili's home in the Belgian capital.

Prosecutors have not named the Gulf state implicated in the investigation, although it has been reported widely that it concerns Qatar.

Qatar is staging the ongoing FIFA World Cup, having been controversially awarded hosting rights in 2010.

The tournament has faced major criticism from human rights campaigners because of the country's treatment of migrant workers, criminalisation of homosexuality and lack of women's rights.

Kaili has recently claimed that Qatar is a "frontrunner in labour rights", and "we don't have the moral right for lectures to get cheap media attention".

She met Qatar's Labour Minister Ali bin Samikh Al Marri shortly prior to the World Cup.

The Qatari Government has denied knowledge of the investigation by Belgian prosecutors.

"We are not aware of any details of an investigation," an official told AFP.

"Any claims of misconduct by the State of Qatar are gravely misinformed."

The Qatar 2022 World Cup is due to conclude next Sunday (December 18).