“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.
“The limits set in drinking water are set there for very good reason.
“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.
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.
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.