The holiday island Ibiza is a potential hotbed for sexually transmitted infections, a survey suggests.
Ibiza is popular with clubbers
Researchers questioned 1,500 young Britons who travelled to the island between 2000 and 2002.
They found one in four men and one in seven women had sex with more than one person during their stay. Many failed to use condoms.
Writing in the STI Journal, they said urgent action is needed to protect the sexual health of Britons abroad.
The number of people being diagnosed with STIs, like syphilis and gonorrhoea, in the UK has rocketed in recent years.
There has been anecdotal evidence that many people have contracted infections abroad.
Professor Mark Bellis and colleagues from the Centre for Public Health at Liverpool John Moores
University interviewed Britons between the ages of 16 and 35 at Ibiza Airport at the end of their holiday.
They found that half of those who had travelled to the island without a partner had sex during their stay. Just 60% said they always used a condom.
Many of those questioned said they had slept with more than one person during their holiday.
Some 11% of men and 3% of women had sex with six or more partners.
Almost 9% said they had sex from someone who was not from the UK.
The researchers found people who had a string of sexual partners in the six months before travelling to Ibiza were most likely to have unprotected sex with more than one person during their stay.
"Their behaviour is a significant risk factor in the national and international transmission of STIs," they said.
Almost one in five of those questioned said the main reason they travelled to Ibiza was for sex.
The researchers said the findings show many young people were putting themselves at risk of catching a STI in Ibiza.
"Substantial numbers of individuals visiting international nightlife resorts have unprotected sex with people they meet while abroad," the researchers said.
"This poses an increasing threat to the sexual health of UK residents but as yet little attention has been paid to developing interventions that might reduce sexual risk taking among young people holidaying abroad."
They added: "Without such measures to protect the health of young people abroad, already elevated rates of STIs and unwanted pregnancies in the United Kingdom may continue to climb."
Public Health Minister Melanie Johnson said she was concerned by the findings.
"We are concerned about the levels of unprotected sex highlighted by this survey but it does confirm that we
are targeting our Sex Lottery campaign at precisely the right people.
"In the summer of 2003, we worked with the three main youth holiday companies and Sex Lottery materials, communicating safe sex messages, were shipped into resorts in Ibiza, Rhodes, Majorca, Cyprus and Corfu.
"Materials reminding young holidaymakers to these destinations of the importance of using condoms and practising safe sex were also included in their ticket inserts."