Figures released by the Office for National Statistics (ONS) show that out of just under 800,000 women who conceived in England and Wales in 1997, 400,000 were unmarried. In 1986, by comparison, 305,000 conceptions out of a total of 819,000 took place outside marriage.
Under-age pregnancies fell by 5 per cent in 1997 to 8.9 conceptions per thousand girls aged 13-15, compared with 9.4 per thousand in 1996. Overall, teenage conception rates fell by 1 per cent over the same period, according to the spring issue of Population Trends, the publication of the ONS.
Despite this fall, the figures show that the United Kingdom has the highest number of births per 1,000 women aged 15-19 out of eight European countries studied by the ONS.
In 1997 there were 30 births per 1,000 women aged 15-19 in the UK, compared with 21 per 1,000 in Portugal and 17 per 1,000 in the Republic of Ireland.
The report shows a large increase in the number of women over 35 conceiving in the period since 1991.
In 1997 there were 41.2 conceptions per 1,000 women aged 35-39, and 8.7 conceptions per 1,000 among women aged 40 and over - the only two age groups to register an increase in fertility rates since 1996.
Karen Dunnell, co-editor of the report, said she thought the rise in conceptions was the result of improved educational prospects for women.
She said: "The proportion of women who gain educational qualifications is rising, and their economic activity rates are rising relative to young men's - this is probably one of the main reasons."
The report also found that two-thirds of total conceptions outside marriage resulted in motherhood, compared with more than 90 per cent of conceptions within marriage.
The number of people cohabiting in England and Wales will double by 2021, according to the ONS. About 1.56 million unmarried couples were living together in 1996, a figure expected to rise to nearly 3 million within 25 years.
The ONS estimates the most dramatic increase in numbers of cohabiting couples will take place among the over-35s. It is estimated that by 2006 less than half of all adults will be married.
Divorce rates dropped in 1997, with 146,339 decrees absolute in England and Wales, 6.6 per cent fewer than in 1996.