Sandals, Eurotunnel and Disney have been named among big brands unwittingly funding Islamic extremists, white supremacists and pornographers by advertising on their websites.
Advertisements for hundreds of large companies, universities and charities appear on hate sites and YouTube videos created by supporters of terrorist groups such as Islamic State and Combat 18, a violent pro-Nazi faction, according to an investigation by The Times.
The practice is likely to generate tens of thousands of pounds a month for extremists.
An advert appearing alongside a YouTube video typically earns whoever posts the video $7.60 for every 1,000 views. Some of the most popular extremist videos have more than one million hits.
Big advertising agencies, which typically place commercials on behalf of clients, have been accused of pushing brands into online advertising to boost their own profits.
Analysis by the newspaper of online extremist content reveals that blacklists designed to prevent digital adverts from appearing next to it are not fit for purpose.
Sandals Resorts is advertised next to a video promoting al-Shabaab, the East African jihadist group affiliated to al- Qaeda.
A Sandals spokeswoman said that it made “every effort” to stop its adverts appearing next to inappropriate content. It said that YouTube had “not properly categorised the video” as sensitive.
Commercials for HSBC, Eurotunnel and JD Sports appear on “alt-right” and Islamist websites, including one promoting a “Holocaust Amnesia Day”.
Adverts for John Lewis, Dropbox and Disney are embedded in sunnah-online.com. The website hosts lectures by Abu Ameenah Bilal Philips, a preacher banned from Britain who has argued that a husband cannot be charged with rape, and Esa al-Hindi, a terrorist sentenced to life imprisonment.
Many of the companies said that they were unaware of and “deeply concerned” by their presence on the sites.
They blamed programmatic advertising, a system using complex computer technology to buy digital adverts in the milliseconds that a webpage takes to load.
Many advertising agencies have their own programmatic divisions, which often apply mark-ups to digital commercials without the brands’ knowledge.
Hicham Felter, a spokesman for ISBA, the trade body representing Britain’s biggest advertisers, said: “Programmatic advertising is a big concern for us and the whole advertising industry.
“There is a greater risk of ads appearing in violent, pornographic, extremist and other ‘unsafe’ brand environments because of the volume and speed at which programmatic trading is carried out.”
He added: “The suspicion is that the surge in programmatic trading is being fuelled by the profit media agencies can make rather than because it delivers better results for their clients.”
A spokeswoman for YouTube owner Google said that it had a “zero-tolerance policy for content that incites violence or hatred”.
Advertisers could choose not to appear against content they considered inappropriate, she said.
The six top advertising agencies each denied any wrongdoing, conflict of interest or sharp practice and said that their relationships with clients were transparent.
A Google spokesperson told Travel Weekly: “When it comes to content on YouTube, we remove flagged videos that break our rules and have a zero tolerance policy for content that incites violence or hatred.
“Some content on YouTube may be controversial and offensive, which is why we only allow advertising against videos which fall within our advertising guidelines.
“Our partners can also choose not to appear against content they consider inappropriate, and we have a responsibility to work with the industry to help them make informed choices.”