More than a third of Premier League football fans say they regularly watch matches live online via unofficial streams, according to a BBC survey.
The poll suggests younger adults are most likely to say they stream matches via unauthorised providers.
Nearly a quarter of all fans surveyed regularly watch matches online via special technology, such as Kodi boxes.
Sky and BT Sport hold the live rights for Premier League football, and unofficial streams are illegal.
The Premier League says the law is catching up with pirates and that it will continue to protect its copyright.
- Details of the survey can be heard in full from 10:00 BST on BBC Radio 5 live
According to the poll of 1,000 people for 5 live Daily:
- Nearly half of fans say they have streamed a match online through an unofficial provider - just over a third do so at least once a month and about one in five at least once a week.
- The main reasons include a friend/family member doing it and they just watch; the quality of the stream; and because sports TV packages are considered not good value for money.
- Just under a third of fans do not know whether it is illegal to stream live Premier League matches online from unofficial providers, but another third believe it is always illegal.
In April, a ruling by the European Court of Justice put pirated streams on the same legal footing as copyright-infringing downloads, making it illegal to watch them.
Kieron Sharp, director general of the Federation against Copyright Theft (Fact), said: "People need to be aware that this is no longer a grey area, in fact it is very black and white.
"If you are accessing content for free such as sport, TV and films for which you'd normally need a subscription, or go to the cinema, or buy a DVD, this is illegal.
"As the old saying goes, if it looks too good to be true, then it probably is."
The Premier League is seeking to protect the value of its TV deal, after Sky and BT Sport paid a record £5.136bn for rights to show live matches for three seasons.
Last season saw the biggest drop in live Premier League TV viewing figures for seven years, hitting both Sky and BT. Sky saw a 14% drop, while viewing of BT channels dropped by 2%.
This may have been caused in part by the relegation of well-supported teams such as Newcastle and Aston Villa, while Sky said it was also encouraged by the use of alternative legal platforms such as Sky Go and Now TV.
Earlier this year the Premier League announced what it described as its biggest ever anti-piracy crackdown, focusing on those who provide the streams and supply equipment.
In March, a High Court judge granted an order for the UK's four biggest internet service providers to block access to online servers, making it harder for pirates to switch streams when they are shut down.
A Premier League spokesman said: "Fans should know that these pre-loaded boxes enable pirate broadcasts of Premier League football, and other popular content, and are illegal. People who supply them have been jailed or ordered to pay significant financial penalties.
"We are increasingly seeing prominent apps and add-ons being closed down as the law catches up with them, leading to consumers being out of pocket.
"The Premier League will continue to protect its copyright, and the legitimate investment made by its broadcasting partners. Their contribution allows our clubs to develop and acquire players, invest in facilities and support the wider football pyramid and communities - all things that fans enjoy and society benefits from."
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.
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.
In March, a man from Hartlepool was given a suspended prison sentence and ordered to pay £250,000 for selling Kodi boxes to pubs and clubs.
For the BBC 5 live survey, ComRes interviewed 1,000 adults who report being regular viewers of Premier League football, online between 7 and 15 March.
What were the survey's results?
- 36% of supporters said they streamed live Premier League matches online through an unofficial provider at least once a month, and 22% at least once a week.
- 47% of fans have watched a match through an unofficial provider at least once in the past.
- Younger fans (aged 18-34) are considerably more likely than their older counterparts to say they stream live football matches online through an unofficial provider - 65% do so at least once a month compared to 33% of 35-54 year olds and 13% of those aged 55+.
- Of those fans who stream matches illegally, the most popular reasons are because a friend/family member does it and they just watch (29%); because the quality of online streaming is good (25%) and because sports TV packages are not good value for money (24%).
- 12% per cent of Premier League fans think it is legal to stream games online (not through an official provider), while 34% think it is always illegal and 32% don't know; 4% believe it is not breaking the law but Sky or BT could fine you if they find out, 7% think it is sometimes illegal, and 10% per believe it is legal to watch but illegal to upload a stream.