Research has established that babies are more likely to have genetic mutations if their mothers are exposed to cigarettte smoke from their fathers - or other people - during pregnancy.

The mutations occur in a gene linked to childhood cancers such as as leukaemias and lymphomas.

Exposure increases cancer risk

It is the first hard evidence that passive smoking causes genetic damage in unborn children and has led scientists to call for tougher rules on smoking in public places.

The researchers said: "Some studies suggest that children whose mothers smoked during pregnancy are not at an increased risk of developing cancer as a child."

However, the researchers said that not only is this smoke dangerous, so is that of others.

In the absence of the mother smoking, "there is accumulating evidence that maternal exposure to passive smoke" increases the risk of developing childhood cancer, they said.

The Cancer Research Campaign said that the finding made a clear case for improved health education.

Mounting evidence

Jean King, head of education for the campaign, said that people who were about to become parents ought to be aware of the risks.

Mothers-to-be should stop smoking

Mothers-to-be should not only give up smoking themselves, but they should ensure nobody smoked around them, she said.

"The evidence is mounting all the time of the risk to children and babies in the womb.

"There needs to be more work on cessation - we need to encourage adults o stop smoking before they have children.

"It is really the only responsible thing to do for the health of your children."

Dr Barry Finette, a paediatrician at the University of Vermont in the US, led the research.

It studied 24 newborn babies. Half were born to women who had been exposed to passive smoking, half were born to women who had not.

It found mutations in the white blood cells of the babies born to passive smoking women.

Although it was a relatively small study, the researchers said that their findings were as valid as those from much larger surveys.

Early findings

They also said that, because of the size of the project, their findings could be the first in a series of such discoveries.

"Given our small sample size, there may well be other differences that we were unable to uncover."

Dr Gabriella Sozzi, a specialist at Italy's National Cancer Institute, called for increased protection for pregnant women in an editorial in the journal.