Websites alleged to be offering pirated material are to have their advertising revenue disrupted in a ground-breaking initiative.

Users will be warned that a site is under criminal investigation in a "pop-up" banner police message replacing legitimate paid-for advertising on sites suspected of offering copyright-infringing content.

The scheme is run by the City of London police intellectual property crime unit (Pipcu) with Project Sunblock, a technology firm used by major brands to stop their advertising appearing next to content such as suspected pirated material and pornography.

The project is the latest phase of Operation Creative, aimed at preventing websites from providing unauthorised access to copyrighted material, led by Pipcu in partnership with the creative and advertising industries.

Detective Chief Inspector Andy Fyfe, head of Pipcu, said: "Copyright infringing websites are making huge sums of money though advert placement, therefore disrupting advertising on these sites is crucial and this is why it is an integral part of Operation Creative.

"This work also helps us to protect consumers. When adverts from well known brands appear on illegal websites, they lend them a look of legitimacy and inadvertently fool consumers into thinking the site is authentic."

Brands may not be aware of where their advertisements are appearing online as syndication networks are often used to place advertisements on websites.

Project Sunblock serves a police warning when a website is on the Infringing Website List (IWL) and tries to display an advertisement.

The sites where the police messages appear will have been contacted by Pipcu beforehand and offered the chance to co-operate, City of London police said.

Duncan Trigg, Project Sunblock chief executive, said: "Protecting brands online is at the heart of what we do, so we're delighted to be selected to help the police tackle online piracy and bring about a safer market place for advertisers in the UK.

"Without realising it, advertisers are allowing their brands to be associated with illegal sites, and regrettably, this happens more often than it should. But each time it does, brands are effectively putting money in the back pocket of criminals.

"As advertisers funnel more money into online spend, initiatives like this are crucial to safeguarding their brands as well as their budget."