More than a third of young people are putting themselves at risk of catching sexually transmitted infections, researchers said today.
The survey of 16 to 24 year olds found that 38% of those polled do not always use a condom with a new partner.
The most common reason (44%) for not using a condom was that the girl was on the pill, which offers no protection against sexually transmitted infections (STIs). The second most common reason (17%) was being too drunk.
Experts blamed the situation on poor sexual health education, which led to young people lacking even "very basic" knowledge about sexually transmitted infections (STIs).
Jamie Hardie, president of the Society of Sexual Health Advisors, said misconceptions about sexually transmitted infections were still widespread among young people.
He said: "Many young people lack even very basic knowledge. They don't know what sexually transmitted infections are or how you get them."
The online survey of 19,250 16-24-year-olds found that their main worry regarding sex was pregnancy (41%). This was substantially more than the number whose prime concern was catching HIV (24%) or another sexually transmitted infection such as genital warts or chlamydia (22%).
The study also found that nearly a third (30%) of those polled lost their virginity before the age of consent.
More than half (57%) said they had had a one-night stand, while 23% reported that they would never have one.
Almost half (43%) of those polled had had at least five sexual partners, while nearly a fifth (18%) had had 10 or more.
The Department of Health said the survey, conducted by BBC Radio 1, 1Xtra, MTV and condom manufacturer Durex, showed that there was a need to encourage safer sex among young people.
Health minister Norman Warner said: "We have to get them to use condoms more in their sexual encounters."
But sexual health experts warned that current NHS deficits were causing cutbacks in STI treatment for young people.
Earlier this month a government advisory group warned that a £300m programme to tackle England's epidemic of sexually transmitted infections was in danger of collapse because NHS trusts have siphoned off the money to reduce their debts.
Jamie Hardie warned that these cuts were hindering efforts to improve the treatment of STIs for young people.
He said record NHS deficits had led to cutbacks in the national Chlamydia screening programme, which aims to encourage sexually active under-25-year-olds who would not otherwise come forward to be tested for the most common STI.
Mr Hardie, a senior sexual health advisor in Newham, east London, said the programme, which was meant to cover the whole of England from April, was "being axed all over the place".
"It's been put on hold in Newham and Tower Hamlets and other London boroughs as well as many other areas across the country."
Ann Furedi, chief executive of BPAS - formerly known as the British Pregnancy Advisory Service - said access to sexual health and family planning services should be made " much simpler and easier".
She said: "At the moment this can be hampered by lack of information or awareness, or in some parts of the country, 'postcode providing' and NHS spending cutbacks in sexual health."
