Children are stumbling upon pornography online from as young as seven, a report has indicated.
The survey, from the British Board of Film Classification (BBFC), suggested three-quarters of parents felt their child would not have seen porn online but more than half had done so.
Youngsters under the age of 10 described feeling "grossed out" and "confused" by what they had seen.
The UK is trying to make it harder for children to see adult content.
It is bringing in a new regime of age verification, under which websites hosting mainly pornography will be required to stop UK users from accessing content unless they can prove they are over 18.
Every time a UK IP address attempts to access a pornography website, the user will be required to verify their age.
The plans, part of the Digital Economy Act, were due to come into force in July but have since been delayed for a further six months.
The BBFC has been appointed as the age-verification regulator and will monitor adult sites to ensure they have appropriate means of checking the age of visitors.
David Austin, chief executive of the BBFC, said: "Pornography is currently one click away for children of all ages in the UK, and this research supports the growing body of evidence that it is affecting the way young people understand healthy relationships, sex, body image and consent.
"The research also shows that when young children - in some cases as young as seven or eight years old - first see pornography online, it is most commonly not on purpose."
The report also looked at the effects of pornography on youngsters. Just over 40% of those who knew about pornography agreed that watching it made people less respectful of the opposite sex. Girls spoke of their fear that aggressive depictions of sex would be seen as normal by young males and copied in real life.
The government and the BBFC is not prescribing how sites verify age but it will be done using a variety of methods, including credit card checks and systems such as AgeID, which requires people to upload scans of their passports or driving licences.
Critics say those determined to get around the rules will find it relatively easy to bypass the restriction. And it will remain legal to use virtual private networks which can make it seem like a UK-based computer is located elsewhere in order to avoid the blocks.