The pop-up notices are being placed by the City of London Police IP Crime Unit (PIPCU). They inform viewers that the website is under investigation, and advise that they leave it.
The initiative is part of Operation Creative and uses technology provided by Project Sunblock.
The notices will be included on sites that have been reported by rights holders and evaluated by PIPCU officers.
In the first instance, the site operator is contacted and invited to operate legally. If it does not, further action may follow.
The UK initiative mirrors that undertaken by the National IP Rights Coordination Center against IP-infringing sites in the US.