"I am particularly concerned that a significant number among the approximately 300,000 migrant workers in Bahrain become victims of human trafficking," said Sigma Huda, the United Nations special rapporteur on human trafficking, in a statement after a visit to the country.
Huda noted that about 50,000 of the migrant workers were women.
"These migrants are often lured in their country of origin by unscrupulous recruiting agents with false promises of a certain job or certain working conditions," she said.
"More often than not, they are shocked to find themselves in exploitative situations upon arrival in Bahrain," Huda, a Bangladeshi lawyer, said.
Conditions included physical and mental abuse and 14 to 16 hour working days for seven days a week, while wages were delayed or withheld, passports were confiscated, telephones or access to mail were removed and food was often insufficient.
Huda found that women in particular were "the most disadvantaged" by being excluded from the scope of current labour legislation.
"Many families - a majority I would hope - respect the dignity and human rights of their domestic workers. Some even come to treat their domestic migrant workers like members of their own families," Huda said.
"I am concerned that some... may be deceived prior to their departure for Bahrain about the type or conditions of the activity they eventually have to engage in"
Sigma Huda, UN special rapporteur on human trafficking
"However, other domestic workers are less fortunate and are subject to degrading conditions."
Domestic workers who fled were "frequently re-victimised", while authorities often fail to properly investigate their cases, she said.
Similar conditions prevailed for women from Thailand, Syria, Lebanon, Morocco, central Asia and eastern Europe who ended up in prostitution, according to Huda.
"I am concerned that some... may be deceived prior to their departure for Bahrain about the type or conditions of the activity they eventually have to engage in," she said.
With prostitution criminalised, they face prosecution, detention and deportation while "traffickers can cloak themselves in a suit of legality".
"I am pleased to note that the government has recognised human trafficking as a problem and has been taking measures to address it," Huda said, referring to an anti-trafficking bill, draft labour reform and a recently built safe house.
"[However] much remains to be done for the government to implement Bahrain's international obligations related to human trafficking."
Huda is currently on a mission to three Gulf states, including Oman and Qatar, as well as Bahrain.