Customs minister John Healey said new research showed buying cigarettes from smuggled sources could be "downright dangerous".

He told BBC News: "Smoking is bad enough but the risk to people's health from these fake cigarettes is even greater.

"These have hugely higher levels of tar, nicotine and some of the cancer-causing chemicals lead and arsenic."

He said 85% of cheap cigarettes sold illegally in big cities, and more than half of all smuggled cigarettes seized nationally, were counterfeit.

Fake cigarettes used to be easier to spot as the packets sometimes had foreign writing, did not display health warnings, or spelt a brand name incorrectly, a Customs and Excise officer explained.

But now the counterfeits, which replicate many popular brands, have become much more sophisticated. It is near impossible to tell from the outside packaging whether you're buying fakes or not, even if they have health warnings and a tax stamp.

Tests of fake cigarettes revealed "substantial contamination ... by heavy metals, including the human carcinogens cadmium and arsenic," said Dr. Ed Stephens, a geoscientist from the University of St. Andrews, releasing his independent research.

"This is a worrying find for the health of those who are heavy and habitual smokers of counterfeits."