Trading standards chiefs are probing allegations Muslim holy water contaminated with arsenic is on sale in Newham.
An undercover investigation found Zamzam water containing high levels of nitrate and potentially harmful bacteria being sold in a shop in Upton Park.
The unnamed store was among several across London and Bedfordshire alleged to be offering the product for sale.
Genuine Zamzam water is taken from a well in Mecca and is not supposed to be exported for commercial use, although tourists are allowed to bring back small amounts for personal consumption.
It is not known whether the water being sold in Upton Park was from the official source, although Saudi officials have said tests show water from the Mecca well is fit for human consumption.
A Newham Council spokesman confirmed the matter was now under investigation, but it is understood the products in question have since been removed from sale.
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