French authorities have put a retreat organised by Australian self-styled guru Ellen Greve under surveillance, the Le Parisien newspaper reports.
The newspaper says the authorities are worried that it is a dangerous cult that has had a role in the deaths of three people around the world.
Jean-Michel Roulet, the head of the French Government's anti-sect unit Miviludes, has told the newspaper that the prolonged fasting preached by Ms Greve is "aberrant".
Mr Roulet says Ms Greve's group has used "a veritable attack on an individual's freedom by way of mental manipulation".
A seminar in the south-eastern village of Devesset headed by Ms Greve, who calls herself 'Jasmuheen', is under "high surveillance".
However, officials say those attending are all adults and that, barring a mishap during the gathering, there are no grounds for police to break it up.
Ms Greve, 48, teaches that people can live almost entirely without food or water under an approach she calls "breatharianism".
She herself claims to have eaten nothing since 1993, surviving only on air and light.
Three people believed to have been influenced by her principles have died.
In Australia, two members of an association close to Ms Greve were convicted after the death of a woman.
Another disciple died in the Scottish highlands in 1999.
And in 1997, the death of a kindergarten teacher was reported in Germany.