Internet traffic to legal pornography sites in the UK comprised 8.5% of all "clicks" on web pages in June – exceeding those for shopping, news, business or social networks, according to new data obtained exclusively by the Guardian.

Only "arts and entertainment" – a category that is boosted by Google's video site YouTube – and search engines were bigger, at 9.5% and 15.7% respectively.

The figures, which do not include traffic from mobile phones, were compiled by SimilarWeb, a web measurement company based in Tel Aviv which tracks clicks online rather than total volume of traffic. Otherwise, the figures would be distorted by the sheer size of video files such as YouTube and the BBC's iPlayer, which is classed under "news and media" because it sits within the BBC's website.

The data does not include illegal searches for child abuse, which typically travel over secret networks such as Tor, or use peer-to-peer technology to try to hide the abuser's identity.

"Traffic on adult sites represents a huge portion of what people use the internet for, not just in the UK but around the world," said Daniel Buchuk, head of brand and strategy at SimilarWeb. "It is astonishing to see that adult sites are more popular in the UK than all social networks combined."

"People don't just 'stumble' upon adult content. More than 8% of Google UK searches led to adult sites in the past three months," he said.

The data was released as internet service providers wrestle with the challenge posed by the prime minister, David Cameron, who earlier this week tasked them with ensuring that within 18 months anyone signing up for internet service would have to choose whether to opt in or out of being able to access pornographic sites.

Internationally, it appears that Britain is far less interested in porn than Germany, where it makes up 12.5% of traffic – nearly half as much again as the UK – and somewhat less than Spain, where it comprises 9.6%.

But Britain ranks above the world average of 7.7%, and just slightly ahead of the US, where adult traffic makes up 8.3% of clicks.

Cameron's anti-pornography policy has been criticised by internet experts who say it will be ineffective and commentators who point to his refusal to take action over the Sun's topless Page 3 pictures as contradictory.

The controversy over the proposed blocking intensified over the week when the website of Claire Perry MP, who has campaigned against access to pornography, was hacked to show adult pictures. It also emerged that the Chinese telecommunications company Huawei operates the HomeSafe filtering system used by TalkTalk, one of the UK's largest ISPs.

Cameron praised HomeSafe when launching his scheme – but it emerged that the system collects data about every URL that customers try to access. The system holds a list of "banned" URLs determined to contain pornographic content; every request via TalkTalk's system is routed through Huawei's system and checked against the list, but requests from customers who have opted to view pornography are ignored.

That, campaigners argue, means there is in effect no "opt out" from the filters – only from their application.

The Institution of Engineering and Technology (IET) said on Wednesday that "protecting children from seeing legal adult pornography … [is] best addressed by parents following the excellent advice provided by Get Safe Online". Martyn Thomas of the IET warned that "universal blocking of websites, search terms and content is a blunt, ineffective tool".

Buchuk said: "The hardest part for the government, which is what companies like Facebook or Tumblr are currently struggling with, will be to draw the line on what gets banned."

SimilarWeb collects its data by watching the activity of people who have installed one of its browser plugins, which Buchuk said were installed in millions of machines worldwide. Around 1% of the internet populations in most western countries use its plugins, he said, allowing a reliable estimate to be made.

The data includes cases where people go to pornographic subdomains within sites such as Tumblr, now owned by Yahoo, but not on YouTube, which includes content restricted to adults which is protected by a login.

Buchuk suggested that the policy of forcing people to choose whether to opt in or out of accessing adult sites could have dramatic knock-on effects: "Other industries such as online gambling often rely on display advertising on adults sites." That could mean lower revenues for both the sites and the gambling companies.

• This article was amended on 29 and 30 July 2013 to clarify that Martyn Thomas represents the Institution of Engineering and Technology (IET).