The growing number of fake cigarettes coming into the UK contain high levels of hazardous substances, the government has warned in a new campaign.
Can you be sure you know what you're smoking?
The fakes are not only illegal but pose an extra health hazard to smokers buying them, warn ministers.
Criminal gangs have the cigarettes made mainly in Eastern Europe and China to look like well-known brands.
More than a million fakes - containing high levels of substances like lead and arsenic - are seized daily in the UK.
The illegal tobacco market grew rapidly during the 1990s, when cigarettes were exported from the UK to avoid tax and then smuggled back in and sold on the black market.
A government campaign managed to stem the supply, but the criminal gangs involved have now switched tactics and started manufacturing fake cigarettes, often sold in pubs, on streets or in markets.
Smuggled goods now make up 15% of the UK cigarette market.
The campaign launched on Wednesday is encouraging smokers to boycott the fake goods.
Customs minister John Healey said new research showed buying cigarettes from smuggled sources could be "downright dangerous".
He told BBC News: "Smoking's 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 London, 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.
In three London hotspots - Holloway Road, Dalston and Whitechapel - officials found all cigarettes tested were counterfeit.
Mr Healey appealed for help from the public in the form of information about where gangs were operating.
Independent research carried out at St Andrews University showed fake cigarettes being sold in the UK contained five times as much cadmium as genuine cigarettes.
Cadmium can severely damage the lungs and is linked with kidney disease.
Fakes also carry nearly six times as much lead, which damages the organs and nervous system, especially in children.
High levels of arsenic, which increases the risks of lung, liver and other cancers, were also found, said Customs.
Further research by an independent laboratory showed counterfeit cigarettes seized by Customs also contained 160% more tar, 80% more nicotine and 133% more carbon monoxide than genuine cigarettes.
The government says any brand can be faked, and many smokers are unaware of the fact cheap cigarettes may not be genuine, nor of the added health risk they contain.