The City of London Police has started the process of getting in touch with websites it believes are making money by breaking copyright laws.
Two sites known to share links to pirated films, games and music have received letters from police and the National Fraud Intelligence Bureau.
In the letters, it is stated that the operators of the sites are in violation of laws which carry custodial sentences.
The recipients have until June 14 to respond.
A City of London Police statement read: "These websites are able to operate and profit from advertising on their sites without having licenses 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."
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