Advertising on some illegal websites has been replaced with a message from the police warning users that the site is under criminal investigation for copyright infringement.
The banners replace paid advertising and instructs users to leave the website.
This is the latest part of Operation Creative, a initiative set up by the Police Intellectual Property Crime Unit (Pipcu) and the creative industries to target online piracy, the BBC reports.
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The adverts are placed onto the sites using technology provided by Project SunBlock, a company that stops legitimate adverts from "appearing alongside questionable content such as pirated material or pornography".
These adverts will work to inform people who are unaware that the site is illegal, but will not deter "committed pirates" Adam Rendle, a senior associate at law firm Taylor Wessing, told PC Pro.
The announcement sparked criticism from social media users who mocked the police's 'hard-line' approach to tackling piracy.
However, the main objective of the operation is to target the revenue these sites receive from advertisers, not simply to warn off offenders.
Piracy sites make a large proportion of their money from advertising, so disrupting this revenue stream performs a "crucial" role in minimising "criminal profits", said Pipcu's Det Ch Insp Andy Fyfe.
The BBC's Dave Lee describes the approach to disrupt their finances as "a more painful way" of affecting the owners of these illegal sites.
Some social media users have questioned why the police would be involved in tackling piracy, which is widely considered to be a civil offence. But piracy committed on a commercial scale is a criminal offence and can be prosecuted under the Copyright, Designs and Patents Act 1988.
Others suggested that media companies may have more success in tackling piracy if they made the legal alternatives more appealing.
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