TUESDAY, Nov. 11 (HealthDay News) -- Six out of every 10 middle-aged Britons do not use a condom during a first-time sexual encounter, a new study shows.

Those numbers might be similar for Americans, one expert said. "Data in the U.S. are likely comparable and, given prevailing policies with regard to contraception, may be worse," said Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine in New Haven, Conn.

In fact, U.S. rates of all STDs, including HIV, have been increasing with an estimated 19 million new cases each year and more than 1 million people living with HIV. Almost half of the new cases of STDs are among people 15 to 24, according to the U.S. Centers for Disease Control and Prevention.

But older adults are at risk, too. "Often it's assumed that sexually transmitted infections are just increasing among young people, but U.K. surveillance data shows that sexually transmitted infection diagnosis rates are on the increase among all age groups in the U.K.," said Catherine Mercer, the study's lead researcher and a lecturer at the Centre for Sexual Health & HIV Research at University College London.

Most Britons engaging in unprotected sex are in their 30s and 40s and in relationships where there is an age difference of five or more years, according to the report, which was published in the Nov. 12 online edition of the International Journal of Epidemiology.

"Low rates of condom use among those starting partnerships in their 30s and 40s means that they too are at great risk of sexual infections," Mercer said.

For the study, Mercer's group collected data on more than 11,000 men and women who participated in the second British National Survey of Sexual Attitudes and Lifestyles. The survey included questions on recent partnerships, condom use and how soon after meeting they had sex.

Among all the people surveyed, almost 9,600 reported having heterosexual sexual partners in the past 12 months. More men (39.1 percent) than women (20 percent) said that these relationships were "not regular," the researchers reported.

More women (55.2 percent) than men (38.9 percent) said their partnerships were marriages, or involved living together with the partner. Men reported having sex sooner after a first date than women. One in five men said they had sex within 24 hours after meeting their partner compared with one in 10 women.

Over all age groups, condoms were used by 55.3 percent of the partners during their first sexual encounter. However, the rate of condom use declined with advancing age. For example, among 16- to 19-year-olds, 68 percent of males and 67.4 percent of females used a condom during a first sexual encounter, while among 35- to 44-year-olds only 38.1 percent of men and 28.8 percent of women used a condom.

In addition, in relationships where there was an age difference of five years or more, 60.8 percent were unlikely to use a condom during a first sexual encounter, compared with 44.1 percent of partners who were closer in age.

According to Mercer, rates of STDs are increasing in the U.K. In fact, the Health Protection Agency found a 6 percent increase in the number of new STDs in 2007 compared with 2006.

Additional research found that in one area of England, the rate of STDs more than doubled between 1996 and 2003.

"Everyone starting a new sexual relationship, regardless of age, should use condoms and continue to do so, until they and their partner have both been tested for sexually transmitted infections," Mercer said.

People ignore condoms at their peril, Katz added.

"Consistent use of condoms is the most reliable way of preventing HIV transmission next to abstinence, and is effective in preventing other transmissible diseases and unintended pregnancy as well," Katz said.

The lack of condom use by mature adults in the British study is noteworthy and disturbing, Katz said. "These are likely people who know about condoms and can get them. There is clearly a need to better educate men and women about the hazards of unprotected sex, and the advantages of condom use. Easy access to condoms should be a priority as well," he said.

More information

For more on STDs, visit the U.S. National Library of Medicine.

SOURCES: Catherine Mercer, Ph.D., lecturer, Centre for Sexual Health & HIV Research, University College London, U.K.; David L. Katz, M.D., M.P.H., director, Prevention Research Center, Yale University Medical School, New Haven, Conn.; Nov. 12, 2008, online edition, International Journal of Epidemiology

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