Paris-based Sherpa said it had asked the public prosecutor of Nanterre, a suburb of Paris, to investigate its findings that Vinci's Construction Grand Projets division and the managers of its 49 percent-owned Qatari unit QDVC were involved in "forced labour" and "keeping people in servitude".
Vinci, which employs 3,500 people in Qatar and counts the wealthy Arab state as its second-largest shareholder, said it would sue Sherpa for libel and that the claims "severely" hurt its image.
The rest of QDVC belongs to Qatari Diar, an arm of the Qatar Investment Authority (QIA), which holds 5.3 percent of Vinci's shares. QIA declined to comment.
Sherpa's Laetitia Liebert told Le Parisien in an interview that one of the financial crime-focused organisation's lawyers travelled to Qatar and found that migrants worked 66 hours a week and had their passports confiscated.
"Vinci absolutely denies the claims made by Sherpa. The group respects local labour laws and fundamental rights in Qatar as well as in all the countries where it operates," a Vinci spokesman said by telephone.
"In Qatar, each QDVC collaborator has free access to his passport while work and rest times are strictly respected," spokesman said.
Allegations of forced labour in Qatar come up frequently.
Sherpa wants France to pass legislation forcing big international groups to be "vigilant" about human rights violations made by their units or suppliers operating abroad, Liebert said.
Vinci noted that it belonged to the United Nations Global Compact under which companies agree to the organisation's 10 principles regarding human rights, labour, environment and anti-corruption.
Pierre Gattaz, the head of the French MEDEF employers association, said any effort along the lines proposed by Sherpa should be an international one.
"All countries must fight so that the situation improves everywhere in the world," Gattaz told the Europe 1 radio.
According to the second annual Global Slavery Index released last year by Walk Free Foundation, an Australian-based human rights group, 29,400 people, or 1.4 percent of Qatar's population, are estimated to be working as slaves, in forced labour or domestic servitude.
Qatar's authorities have denied these claims and dismissed them as a campaign against the first Gulf nation to host a Soccer World Cup. Qatar has also said none of the workers employed for World Cup projects have been exploited.
(Reporting by Dominique Vidalon, additional reporting by Amena Bakr in DOHA, editing by Andrew Callus and Louise Heavens)
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