One in four sexually active Brit youngsters, despite awareness, fails to use any form of contraception with a new partner.
The staggering figure has moved health campaigners to urge the government to give sex and relationships education in all schools a higher priority.
The survey said that personal hygiene was rated as the most important consideration, ahead of contraception, when preparing for a date that could lead to sex.
Marie Stopes International carried out the worldwide survey for World Contraception Day on September 26.
The UK results revealed 16 percent of young people who had unprotected sex said they did not use contraception with a new partner as they forgot.
Meanwhile, 13 percent said their partner preferred not to use contraception and 19 percent said they had been drunk and forgot.
"We are calling on the Government to put sex education back on the agenda and ensure that all schools, including faith schools, teach a standardised curriculum to all 15-year-olds," the Sun quoted Tracey McNeill, vice-president of Marie Stopes International, as saying.