Browsing some of the internet's most popular pornography websites is increasingly putting visitors at risk, research has found.

Advertisements displayed by the sites, which are visited by millions every day, were found to be installing harmful files without users' knowledge.

Researcher Conrad Longmore found that two popular sites - xhamster and pornhub - posed the greatest risk.

He said it should be easier for users to report malicious advertising.

He said it was primarily Windows users at risk, but that criminals were increasingly turning their attentions to mobile devices.

While none of the porn sites observed hosted any malware themselves, it was the embedded advertisements within their pages that created problems for users.

"We call these malicious advertisements 'malvertising'," explained Mr Longmore.

"The way the ads are bought and sold across all websites is incredibly complex.

"Ads can often be repackaged and resold so that it is hard to tell where they originated from, and the criminals behind them go to great lengths to disguise what they are doing."

'Sudden spike'

Mr Longmore compiled his figures using Google's diagnostic advice service, which regularly analyses websites for harmful content.

The data showed that xhamster - listed by monitoring firm Alexa as the 46th most popular site on the internet - had malvertising on 1,067 out of 20,986 pages (5%) screened in the past 90 days.

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According to Alexa's statistics, the average user of xhamster would look at 10.3 individual pages - meaning a potential 42% risk of stumbling across harmful adverts in each viewing session.

Another site, pornhub, was found to have dangerous advertising on 12.7% of its pages.

Mr Longmore said: "There seems to have been a sudden spike in malware on popular sites, especially in the past week or so. "

However, the web's most popular porn site, xvideos, was not found to have any harmful adverts in the time sampled by Google's system - a sign the site had "cleaned up".

Reporting mechanism

Mr Longmore believes a culture of users being afraid to "kick up a fuss" means many instances of malvertising go unreported.

"Part of the problem is that porn is a taboo subject," he said.

"But the reality is that these are hugely popular sites with many of them in the top 100 most popular sites globally. Some of them pull in more traffic than the BBC, so this is potentially a very big issue.

"Site operators could put a quick reporting mechanism on their sites to flag up bad ads and other concerns, and ad networks should also take some responsibility here.

"I don't see that happening any time soon, and perhaps the best thing that users of these sites can do is ensure that their machines are up to date."

The BBC has approached the owners of xhamster and pornhub but has yet to receive any comment.