Psychologists surveyed 620 women and found that those whose partners stopped smoking with them were much more likely to reduce their tobacco intake or stop.

Dr Peter Appleton, of the University of Wales, and Professor Peter Pharoah of the University of Liverpool, questioned women who were smoking at the beginning of their pregnancy and after they had had their children.

They found that a third of the women's partners reduced or quite smoking and this had a big impact on the women.

Dr Appleton said: "Addiction is not the only reason women find it difficult to quit. Having other people around who also smoke - partners, family members, friends, workmates - is a known risk factor for continued smoking."

The survey, published in The British Journal of Health Psychology, found that partner smoking was independent of social class, emotional support given by the partner and joint planning of the pregnancy.

One in six

Dr Appleton advised that couples should consider trying to give up smoking together when the woman becomes pregnant.

This would help the father play a greater role in the pregnancy. "That way dad helps both his partner and the unborn baby," said Dr Appleton.

About a third of women of child-bearing age smoke. According to a recent study by the Health Education Authority (HEA), only one in six quit when they become pregnant and the numbers have not changed over the past five years.

Despite the danger to their unborn children, women find it hard to kick the nicotine habit

The HEA also found a link between social deprivation and smoking.

Young women who were unemployed, working in manual jobs or single were more likely to smoke than others.

This is despite warnings about the danger to their children. The babies of women who smoke are more likely to be underweight and premature than those who do not smoke and studies have shown a link between childhood cancer and women who cannot quit during pregnancy.