Bottled water that claims to come from Mecca has been found to contain high levels of cancer-causing chemicals.
Zam Zam is sold as coming from a sacred well in Mecca
People have been told not to buy Zam Zam water after tests showed three times the permitted level of arsenic.
The water is advertised as coming from the sacred well of Zam Zam in Mecca, which is the most holy city in Islam, and demand increases during Ramadan.
Westminster City Council said the water cannot legally be exported, so any on sale is unlikely to be authentic.
The tests, carried out by the London council last year during the Islamic festival of Ramadan, found Zam Zam also contained twice the legal level of nitrates, which can affect infants.
"If you do see Zam Zam water on sale anywhere please inform your local Environmental Health team," said a Westminster City Council spokesman.
He added: "This advice does not relate to the genuine Zam Zam water being brought into UK by returning pilgrims, as an accompanied or unaccompanied ¿personal import¿."