The UK advertising watchdog has criticised the lax age verification systems used by websites such as Facebook, after finding that more than 80% of children lie about their age when using social media.

The Advertising Standards Authority uncovered the issue while conducting a survey assessing what kind of ads young people see and whether companies are sticking to the UK advertising code.

The survey found that 83% of the 11 to 15 year olds whose internet usage was monitored registered on a social media site with a false age.

Just over 40% of the children signed in stating they were over 18 years of age, with one even claiming to be 88.

While the report found that advertisers were accurately targeting their ads many children were seeing inappropriate content – such as ads for gambling, alcohol, slimming aids and overtly sexual dating services – because they were lying about their age.

The ASA said that the report "clearly asks questions" of social media owners such as Facebook, which it believes knows that young users are using false ages to log in.

"We will be raising these issues with social media companies," said Guy Parker, chief executive of the ASA. "If advertisers and social media companies know that children say they're older than they are don't they have a crucial part to play?"

Parker said that the ASA intended to talk to social media companies about considering "taking a tougher line" than using the easy-to-bypass self-declaration system.

"We all need to be part of this conversation about how best to set the boundaries within which our children explore the world around them," he added.

The ASA, which commissioned market research firm Actual Customer Behaviour, found that the group of children viewed 427 ads in the time period they were monitored online. Of those 98.4% adhered to the ASA's advertising code.

However because many of the children lied and said they were over 18 years of age, a number of the ads should not have been viewed.

The ASA found that 24 ads were designated as being not suitable for under 18s, for products such as gambling, alcohol, slimming aids and overtly sexual dating services.

Facebook proved to be the most popular site, delivering 385 of the total of 427 ads viewed by the children.

Google-owned YouTube was second with 20 ads, followed by Stardoll (16), Spotify (4) and Twitter (2).

The ASA said that it will consider whether new guidance needs to be issued to advertisers targeting online, and whether "any solutions that have been mooted for other forms of online protection for children might be appropriate in an advertising regulatory context".

In June Facebook was forced to undertake a major crackdown on offensive material after mounting pressure from advertisers such as Nissan, Nationwide and BSkyB, which pulled campaigns over concerns about the content on web pages where they appeared.

"The ASA's report recognises that when advertisers use a service like Facebook they can be confident that their adverts are reaching the type of people they want to," said a spokeswoman for Facebook UK.

"It's particularly useful for brands who have products which are age-restricted. But when children lie about their age, that value and intention can be undermined. It's important for parents, schools, safety organisations – like Childnet and FOSI – and platforms like ours to encourage children not to do this. Simply put, they will have a better experience if they don't lie.

"Technology can help to spot children who do, but there's no substitute for action by people who know the child in the real world. The ASA have highlighted an important issue and we are committed to continue to work with them, our clients and others to address this."

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