A campaign warning people of the health risks of fake holy water appears to be working, health officials have said.
Zam Zam is sold as coming from a sacred well in Mecca
Environmental health officers carried out on-the-spot inspections at about 50 stores to ensure fraudulent Zam Zam water was not being sold.
They said the water was known to contain high levels of nitrates and arsenic, was not being sold.
Westminster City Council said no fake Zam Zam was found during the operation ahead of Ramadan.
The stores inspected were in Edgware Road, Queensway, Lancaster Gate and Hyde Park.
Council officer James Armitage said: "We're delighted that our public health message on fake Zam Zam appears to be getting through.
"But there's no room for complacency and we would urge people to continue to be vigilant, especially during Ramadan."
The water is advertised as coming from the sacred well of Zam Zam in Mecca, the most holy city in Islam, and demand increases during Ramadan.
The warning does not cover genuine Zam Zam, which is sourced from the Well of Zam Zam, located within the Masjid al Haram in Mecca.
Councillor Audrey Lewis was concerned Muslims may be exploited into buying counterfeit Zam Zam during the holy month of Ramadan.
She said: "The Kingdom of Saudi Arabia forbids the commercial export of genuine Zam Zam, so we have no idea of the true source of the water which ends up on the streets of the UK.
"But what we can say with certainty is that any bottles of Zam Zam on sale in the UK could be unsafe and would urge people not to be tempted to drink them, and report any sightings to the authorities.
"It is both immoral and unacceptable to put people's health at risk in this way, particularly during Ramadan, and we are calling on the community to help us stamp out such exploitation."