A French prosecutor has opened a preliminary investigation into allegations that construction company Vinci gravely mistreated migrant workers in Qatar as it prepares to host the 2022 World Cup.

Vinci has vigorously denied the charges. Vinci's Qatar subsidiary QDVC has contracts worth 2.2 billion euros ($2.4 billion) in Qatar.

Activist group Sherpa said Monday that it was informed of the investigation by the prosecutor's office in the Paris suburb of Nanterre. Sherpa filed a legal complaint against France-based Vinci last month, accusing the company of driving workers into forced labor and bondage.

By deciding that Sherpa's allegations warrant investigation, French prosecutors could increase pressure to improve conditions for guest workers, who are playing a significant role in major public works projects in Qatar.