The City of London Police and the National Fraud Intelligence Bureau (NFIB) have started contacting websites suspected of profiting by breaking copyright laws.
Letters have been issued to two sites known to share links to pirated copies of music, movies and games, according to the BBC.
It is not known which sites have received the letters, but both are known to be run outside UK borders.
The letters warn the site operators that they are breaking UK copyright laws and could face heavy jail sentences if they fail to respond by 14 June.
City of London Police said in a statement: "These websites are able to operate and profit from advertising on their sites without having licences or paying the creators and owners of the films, TV programmes, music and publications.
"Intellectual property crime is a serious offence that is costing the UK economy hundreds of millions of pounds each year."
Read more about copyright infringement
- Anonymous uses Aaron Swartz suicide to call for copyright reform
- Protecting your business from copyright infringement
- European copyright ruling good for IT innovation, say lawyers
- UK ISPs block access to pirate sites
- Copyright laws to get a digital facelift
- Digital Millennium Copyright Act (DMCA)
- US anti-piracy group proposes locking infringing computers
The statement said the police action against pirate websites is also aimed at protecting UK citizens from the malicious and harmful software that can be found on these sites.
The NFIB said concern about the harm done by pirate websites to the UK economy had led to an initiative between government and the media industry to target such activity.
Since 2010, content providers have used copyright law to target pirate sites through court orders to cease operation or to force internet service providers (ISPs) to block access.
The decision followed the publication of a report of more than 600 pages of analysis and recommendations for the future of France’s anti-piracy law and digital policy.