BRITAIN has the world's highest rate of unmarried teenage mothers, according to a report published yesterday.

The first worldwide survey of young women's sexual activity showed that 87% of the 41,700 children of 15 to 19-year-old girls in the UK were born outside marriage, compared to 62% in America and just 10% in Japan.

The figure was also higher than the rate in the Third World, according to the survey of 53 countries, including six from the developed world.

Experts from the New York-based health group The Alan Guttmacher Institute, which compiled the research, estimated that on average up to 30% of girls in the 15 to 19 age group worldwide had sex against their will.

The report comes just two weeks after Government figures revealed Scotland had one of the highest rates of teenage pregnancies in Europe. The Scottish Health Statistics for 1997 showed that pregnancies among girls aged 13 to15 rose by 10% to 916, the highest number for a decade.

The latest survey also revealed that Britain has one of the highest sexual activity rates in the world among women under 20. It found that 87% of women aged 20-24 had sex before they were 20 and 86% were not virgins when they married.

In developed countries, an average of 77% of women have had sex by the time they are 20, in Sub-Saharan African countries the rate is 83% and in Latin America and the Caribbean it is 56%.

The report also found that about 10% of women in Britain are married by the time they are 18, compared with 11% in the US, 24% in Brazil and 38% in Ghana.

The London boroughs of Lambeth, Southwark and Lewisham have the highest number of teenage pregnancies, with 10% of women living in these areas becoming pregnant by the time they are 20.

The report covered 75% of the world's population and was published by the International Planned Parenthood Federation to mark International Day of Families yesterday.

The institute said there were one billion people aged between 10 and 19 around the world today, making it the largest generation of youth in history. A spokeswoman said: ''The 541 million young women, 281 million of whom are aged 10-14, are a formidable demographic force, and their pattern of childbearing has major implications for the size of the world's population.''

At the launch of the report in London yesterday, Jeannie Rosoff, president of the American Institute, said: ''The lives of the young women depicted in this report are to some extent already determined. Most are no longer at school, many are married or mothers, and some have been exploited sexually.

''But the same fate does not need to befall their younger sisters, or the next generation of adolescents. Parents, communities and governments must recognise how quickly the world is changing, and how imperative it is to direct attention to improving the situation of girls and young women.

''Indifference, wishful thinking and denial will not prepare their children, particularly their girls, to take their rightful place in a modernising world.''

Ms Rosoff said she was surprised that most teenage sexual activity in all countries occurred outside marriage.

Anne Weyman, chair of the Family Planning Association, said the survey reflected an already well known trend of later marriage and children outside wedlock.

She said: ''The survey findings reflect the known trend that people are getting married later and that more couples are having children outside marriage than ever before.''

The report was based on demographic health surveys from the participating countries.

Margaret Jones, chief executive of the Brook Advisory Centres, which offer sex advice to teenagers, said: ''What this report shows us is that the majority of young women today do not rush into marriage because they become pregnant.

''Although teenagers are far less likely to have a baby today compared with 20 years ago, they are more likely to have a baby outside marriage. This reflects the trend away from the shotgun marriage, which carried a high risk of divorce, towards cohabitation.

''In 1995, 67% of babies born to teenagers outside marriage were jointly registered by both parents, suggesting that the majority of teenage mums are nonetheless in stable relationships. However, the UK does still have one of the highest teenage conception rates in Europe.''