It suggests that one in six parents have not discussed sex education with their children. One in 20 say they have no intention of broaching the subject.
One in four have not talked about sexually transmitted infections or HIV and Aids.
The survey found that while 90% of parents believed sex education was best done by them, many are embarrassed and uncomfortable to tell their children about the facts of life.
One in five parents said they did not feel equipped with the information they needed to talk to their children.
The survey, carried out for the sexual health charity Marie Stopes International (MSI), revealed that one in five parents believed their children would find out about sex themselves.
However, just 9% of parents believed schools should be the main source of advice about sex and relationships.
Only a third of parents with seven-year-old children had talked to them about sex, rising to 43% when the youngsters were 12 and 58% of parents with a 15-year-old child.
The poll found that 17% of people with a 15-year-old child were still only "intending" to discuss sex with their teenager.
Research has shown that one-third of under 16's are sexually active and every year 15,000 girls under 18 have an abortion.
'Never too early'
Earlier this year the Family Planning Association launched a video for parents saying it was "never too early" to talk to children, and advising that sex education should start as young as seven.
Family campaigners have opposed the stance, saying talking to children too young encourages promiscuous behaviour.
The government last year launched a £65m campaign aimed at reducing the UK's rate of teenage pregnancies, which is the highest in Western Europe.
And new guidelines for the teaching of sex education in schools were published earlier this year.
The MSI has launched a "Sexplanations" booklet aimed at helping parents to discuss the subject with their children.
Dr Liliana Risi from MSI said: "The survey reveals a gap between good intentions and the reality of what parents will or will not discuss.
"Even when parents are discussing sex they tend to focus on the emotional side.
"This is obviously very important but young people also need hard facts and basic non-judgmental information so that they can protect themselves from unwanted pregnancies and sexually transmitted infections, and lead safe, happy sexual lives."