BBC News 24's James Westhead reports

The team in Minneapolis found traces of a chemical called NNK in the urine of smoking mothers' babies.

NNK comes from nicotine and is one of the worst carcinogens in tobacco smoke.

Babies of smokers were tested

Professor Gordon McVie, Director-General of the UK Cancer Research Campaign, said: "This is absolute dynamite.

"Here we have cast-iron, watertight evidence that the baby is exposed to carcinogens thanks to the mother's smoking habits."

Researchers tested 31 samples from babies born to mothers who smoked during pregnancy.

Dr John Toy of the Cancer Research Campaign: "It's a robust finding"

And of those 31 samples, 22 contained NNK.

Although smoking is known to increase the risk of having a low-birthweight baby, and also increases the risk of losing a baby to sudden infant death syndrome (SIDS or cot death), up to 61% of women who smoke do not quit during pregnancy.

Professor Stephen S. Hecht, who conducted the research, said: "Hopefully this will deliver the message one more time about how dangerous it is for pregnant women to smoke."

Long-term studies

Professor Hecht: Smoking harms unborn babies

Prof Hecht, from University of Minnesota, said more research was needed to determine the likelihood of NNK leading to cancer in the newborns later in life.

Because most mothers who smoked during pregnancy would continue to smoke after they give birth, those children born with NNK were likely to be exposed to years of tobacco smoke.

He said: "We need long-term studies by epidemiologists to determine the consequences of exposure to tobacco smoke."

Clive Bates, director of Action on Smoking and Health (Ash), said: "Babies exposed to cancer-causing chemicals in the womb are suffering from one of the nastiest forms of passive smoking.

"Everything possible should be done to help pregnant women quit smoking."