Muslims in Leicestershire have been urged to avoid a bottled water which may contain high levels of the poison arsenic.
Zam Zam holy water is drawn from a well in Saudi Arabia and cannot be exported from the country for commercial sale.
The county council's trading standards said any water on sale that is labelled as Zam Zam water is of unknown origin and poses a risk.
The water is traditionally drunk by pilgrims for its healing properties.
David Bull, Leicestershire County Council head of trading standards, said: "Food Standards Agency's tests have shown that some 'Zam Zam' water sold in the UK, or brought into the UK for personal consumption, contains high levels of arsenic - almost three times the legal limit.
"Nitrates have also been found, which may increase the risk of cancer."
The penalty for selling false Zam Zam water is an unlimited fine or two years imprisonment.