Public Health England (PHE) has now issued a London-wide health alert about the chalk, otherwise known as Argile, La Craie, Mabele, Nzu or Shiley, which is often used by pregnant women in Asian and African communities as a nutritional supplement and as a morning sickness “antidote”.

Pregnant women are being warned against using a potentially poisonous chalk product to combat morning sickness, because it may contain lead and arsenic.

Environmental health officers seized the product, known as Calabash Chalk, from a premise in Hackney, where it is believed to be on sale for consumption.

Tests showed the seized product contained high levels of the toxic metals, lead and arsenic.

Lead exposure during pregnancy can cause reduced birth weights, impaired neurodevelopment and impaired intrauterine growth.

Negative effects on a foetus or a growing child can be more marked, as their growing bodies absorb Pb more readily, and do not have the same capacity to excrete heavy metals as efficiently as an adult.

Public Health England (PHE) – which came into being on April 1 as part of the National Health Service reorganisation - has now issued a London-wide health alert about the chalk, otherwise known as Argile, La Craie, Mabele, Nzu or Shiley.

It is often used by pregnant women in Asian and African communities as a nutritional supplement and as a morning sickness “antidote”.

A spokesman said it is thought the product may be on sale in a number of London locations.

“The fear is that it is a London-wide issue, but it is difficult to find where it’s being sold and who is using the product,” he added.

The Food Standards Agency (FSA) has issued advice the products should not be eaten, especially by pregnant and nursing mothers, because of potential toxicity.

Dr Yvonne Doyle, regional director for PHE London, said: “If you are suffering from severe morning sickness during pregnancy you should speak to your GP or midwife who will be able to advise you on safe treatment.

“We strongly advise against taking any medicinal or “remedy” product while pregnant without talking to your GP or health visitor about the health risks.”