“Zam Zam” water is taken from a well in Mecca and is considered sacred to Muslims, but samples from the source suggested it held dangerous chemicals.
Tourists can bring back small amounts from Saudi Arabia, but it cannot be exported for commercial use.
An undercover researcher found large quantities of bottles being sold in east and south London, and in Luton.
The president of the Association of Public Analysts said he would “certainly would not recommend” drinking it.
A BBC investigation discovered “Zam Zam” water was being sold by Muslim bookshops in Wandsworth, south-west London, and Upton Park, east London, as well
as in Luton, Bedfordshire.
“The water is poisonous, particularly because of the high levels of arsenic, which is a carcinogen,” said Dr Duncan Campbell, president of the Association of Public Analysts.
“Once the water gets above that limit, it’s not safe.” Secret recordings captured the vendors describing customers who drank it daily. “They depend on it, they don’t drink anything else,” said the owner of an Islamic bookshop in Upton Park.
Last year the Food Standards Agency said people “should consider avoiding” the drink in the UK, which it said came from dubious sources.’Sensitive matter’
The BBC asked a pilgrim to take samples from taps which were linked to the Zam Zam well and to buy bottles on sale in Mecca, to compare the water on sale illegally with the genuine source.
These showed high levels of nitrate and potentially harmful bacteria, and traces of arsenic at three times the permitted maximum level, just like the illegal water which was purchased in the UK.
“Zam Zam” water is gathered from a well in Mecca but there is a ban on exporting it from Saudi Arabia
Dr Yunes Ramadan Teinaz, an environmental health officer who has previously warned about “Zam Zam” water, said it was “a sensitive matter”.
“People see this water as a holy water,” he added.
“They find it difficult to accept that it is contaminated but the authorities in Saudi Arabia or in the UK must take action,” he said.
None of the three shops involved would say why they were selling the water or how they obtained it, but further investigation suggested it had now been removed from their shelves.
The Saudi embassy in London declined to comment on the issue of contamination at the source in Mecca.