Men could soon live longer than women, according to experts.
High levels of drinking and smoking may be to blame
They believe high levels of smoking, drinking and stress may eventually take their toll on women's life spans.
The researchers believe that if current trends continue men born in this century will outlive their female counterparts.
This would represent a complete reversal of the current situation, where women have traditionally outlived men.
Figures from the Office for National Statistics show that men who were aged 65 in 2000 could expect to live to 81, while women could expect to live to 84.
This latest research, which is published in the journal of the Chartered Insurance Institute, shows that the life expectancy of men has increased steadily in recent years.
Male mortality is improving much more quickly than female mortality
Continuous Mortality Investigation Bureau
However, women's life expectancy has increased at a much slower rate and the gap between the two sexes is closing.
"Male mortality is improving much more quickly than female mortality," said Tony Leandro, of the Continuous Mortality Investigation Bureau at the Institute of Actuaries, who led the study.
He said the trend could see men outliving women in the years ahead.
"It is virtually impossible to make accurate projections, but one possible set of outcomes could see baby boys born in the late 21st century having greater life expectancy than baby girls."
He suggested that high levels of smoking among women could be an important factor.
According to Cancer Research UK, almost one in four British adults smokes. This includes 40% of men and 35% of women between the ages of 25 and 34.
I don't think these figures really add up
Professor Martin Jarvis,
Cancer Research UK
However, figures suggest women are smoking more at a younger age and are less likely to give up than men.
"There is evidence that men are smoking less than women, particularly at younger ages and this may be a significant factor," Mr Leandro said.
That view was backed by Richard Willets of the Pension Annuity Friendly Society.
"In recent years, female mortality has not been improving as quickly as male mortality," he said.
"There are two main reasons for this. First, historic patterns of cigarette smoking have been very different for men and women over the past century.
"These patterns have had a long-lasting impact on mortality because the impact of smoking is so significant.
"Second a large proportion of recent improvements has been due to reduce deaths from heart disease.
"This has had much more impact on male rates of mortality because men have far higher rates of heart disease than women."
Stress at work
Adrian Galop of the Government Actuaries Department suggested that the fact that more women are now working may also be a factor.
"The effects of stress and more women in the workplace may also have contributed to the recent decline in the differential."
High stress levels have been linked to heart disease and stroke.
However, Professor Martin Jarvis of Cancer Research UK raised doubts about the findings.
"I don't think these figures really add up," he told BBC News Online.
"There are quite good studies available which show that even in populations where people do not smoke or drink women tend to live longer than men.
"This is really a biological effect. The male is more vulnerable and feebler throughout life from foetus right through to old age.
"The idea that men are going to start living longer than women doesn't add up."
Nevertheless, he said the findings should act as a warning for women.
"It highlights the impacts of health behaviours and particularly smoking," he said.
"It should act as a warning shot across women's bows. If they smoke like men they will be die like men."
Lesley King-Lewis, chief executive of the UK charity Action on Addiction, said: "Women are drinking more than they were ten years ago and this is bound to have an effect on women's life expectancy."
She added: "We are especially concerned about the increasing levels of binge drinking among young women as this has serious social and health repercussions."
The findings follow a warning by one of the government's top doctors earlier this month.
Dr Mac Armstrong, Scotland's chief medical officer, urged women to start living more healthily.
The report showed that 26.1% of women continued to smoke during pregnancy.
It also showed that one in four women binge drink, while 15% exceeded the weekly alcohol limit of 14 units in 1998, a rise of 2% from 1995.