UK police have started replacing ads on websites that provide access to pirated or copyright infringing material with warnings to web users that the site is on a watch list — and a call for them to close the browser page in question.

The initiative, called Operation Creative, is being carried out by the City of London Police’s Police Intellectual Property Crime Unit (PIPCU) and follows a call by the unit, back in April, for advertisers to get behind a plan to tackle IP related crime by helping disrupt piracy sites’ access to ad revenue.

PIPCU also launched a list of offending websites — called the Infringing Websites List (IWL) — which it’s now using as the hitlist for its ad replacing warning banners.

Sites end up on the IWL after PIPCU receives a submission from rightsholders that the site is infringing IP. It then conducts a verification process to determine whether the site should be on the list or not. It says it’s working actively with the BPI, FACT, IFPI and the Publishers’ Association to keep the IWL up-to-date.

“The rightsholders have to put together a very detailed evidential package,” said a PIPCU spokeswoman. “Which is then looked at by the detectives here, and then they make the decision whether to verify if it’s copyright infringing. So it’s a very robust, long process. That way we are confident that the websites on the IWL are copyright infringing.”

The IWL has not being made public, beyond PIPCU sharing it with the digital ad sales traders and networks it’s working with on this operation, namely the IAB UK, ISBA and IPA.

The spokeswoman told TechCrunch the list is not being published in part because the aim is to work with infringing websites to switch them over to a legal path — so it doesn’t want to name and shame publicly when the website in question may subsequently clean up its act.

The list is also dynamic, with new names being added and others taken off as they change their content. At present she said there are around 70 websites named on IWL.

“What we really want to do with this is we’re asking [infringing websites] to engage with us, and we’re giving them enough time to legitimize themselves, correcting their behaviour — it’s kind of a new innovative policing, rather than going and arresting people, it’s a different way of dealing with this stuff,” she said. “If someone is going to correct their behaviour it would be unfair if they had been publicly put out there in the media.”

An earlier attempt by the unit — running a pilot last year — to block mainstream ad network content from appearing on piracy sites resulted in ad space being replaced with malware. The latest move seeks to avoid that consequence by not just blocking content from certain ad networks but replacing banners on sites with police warnings instead.

Commenting in a statement, Detective Chief Inspector Andy Fyfe, Head of PIPCU, said: “If an advert from an established brand appears on an infringing website not only does it lend the site a look of legitimacy, but inadvertently the brand and advertiser are funding online crime. Therefore the IWL also serves as a safety tool, ensuring the reputation of advertisers and brands are not discredited through association with illegal websites.”

Update: Asked whether the ad blocks will also be shown to web users outside the UK, PIPCU provided TechCrunch with the following statement: “This initiative is in collaboration with Project Sunblock, who are an international content verification company, therefore the advert replacement possibly could be seen overseas, however it is more than likely to be mostly UK based.”

On the question of whether PIPCU logs the IP addresses of users who are accessing copyright infringing websites, the spokeswoman said: “As this is a police operation, we cannot disclose all details of the initiative however we can confirm that a number of tactical approaches are used by PIPCU as part of Operation Creative.”