Tackling the use of Kodi and other set-top box software to stream pirated videos is now the top priority for rights-holders, a report says.
Some boxes or "TV sticks" support software add-ons that can stream subscription movies, sport and TV channels over the internet for free.
The Federation Against Copyright Theft (Fact) said about half of its current investigations concerned the devices.
It said boxes configured to receive premium content for free were illegal.
The statements were made in the annual crime report of the government's intellectual property office (IPO).
What are Kodi boxes?
Kodi is free software, built by volunteers, that is designed to bring videos, music, games and photographs together in one easy-to-use application.
Some shops sell set-top boxes and TV sticks known as Kodi boxes, preloaded with the software.
The developers behind Kodi say their software does not contain any content of its own and is designed to play legally owned media or content "freely available" on the internet.
However, the software can be modified with third-party add-ons that provide access to pirated copies of films and TV series, or provide free access to subscription television channels.
"Streaming boxes have steadily increased in popularity in recent years," said Ernesto van der Sar, from the news site Torrent Freak.
"Most use the entirely legal Kodi software, but some are augmented with illegal third-party add-ons.
"They are seen as convenient, as the set-top box format is ideal for the living room.
"Nowadays people often prefer to stream pirated content instead of using traditional torrent sites.
"They see streaming as more convenient and less cumbersome than downloading."
Fact said set-top boxes configured to receive premium content for free were "an emerging threat to the audiovisual industry".
"This is becoming an epidemic," Kieron Sharp, director general of Fact, told the BBC.
"If you are not paying for Sky, BT or one of the pay-TV providers for your subscription channels, you are clearly in possession of an illegal box."
The IPO said the increased availability of such devices presented a "significant challenge".
"We are aware that set-top boxes, while perfectly legal in their own right, are frequently adapted by criminals to illegally receive TV channels protected by intellectual property rights," a spokesman told the BBC.
"The government is working with its partners in industry and with police forces across the country to target criminals looking to profit from this activity.
"We are also working closely with our international partners to target the cross-border infrastructure that underpins illegal streaming."
In August, an investigation by the Police Intellectual Property Crime Unit (Pipcu) led to the arrest of three men who are accused of retransmitting subscription television channels online.
What do the makers of Kodi say?
Some traders sell so-called "fully-loaded Kodi boxes", which are preloaded with third-party add-ons that can access pirated content. These are currently the subject of a legal case.
The developers behind Kodi have said they do not support "piracy add-ons" and have criticised those who advertise "fully-loaded" set-top boxes for sale.
The group said it would maintain a "neutral stance on what users do with their own software", but would battle those using the Kodi trademark to sell a "fully-loaded Kodi box".