Football clubs have potentially made hundreds of millions of pounds selling controversial crypto "fan tokens".
Analysis commissioned by BBC News estimates more than £262m ($350m) has been spent on the virtual currencies.
Some of the tokens are marketed as offering real-world perks to the buyer.
But critics say these perks are insignificant - one offered the chance to vote for songs to be played in stadiums - and clubs have insufficient protection for supporters.
Club-specific crypto-currency
So far, across the five major European leagues 24 different clubs have launched or are considering fan tokens, including eight Premier League sides.
Most offer tokens akin to a club-specific crypto-currency - virtual coins can be bought and sold and their value rise and fall depending on supply and demand.
Some clubs, such as Manchester City, also sell digital collectibles known as NFTs (non-fungible tokens).
Most of the clubs offering fan tokens have signed up to a company called Socios that organises the initial sale and subsequent trading of the virtual coins - but other platforms, including Binance and Bitci, are growing too.
Socios told BBC News it had sold $270m-300m worth of coins through its app. It would not say how much money goes directly to clubs.
Buyers must first convert their money into the company's own crypto-currency, Chiliz.
The research, by crypto-analysts Protos, suggests many buyers are speculatively trading their tokens like other crypto-currencies in an attempt to make money.
But the value of many fan tokens had decreased since they were initially sold by the clubs.
BBC News asked every Premier League team and some major European sides about their plans for, and views on, the new trend.
Only one was happy to comment. A Brighton and Hove Albion spokesman said: "We have not sold these products and have no plans to enter these markets."
Since crypto-currency products are based on a public ledger, known as the blockchain, Protos was able to determine:
Lazio, Manchester City, Porto, and Santos appeared to generate the most sales
the fan tokens to have generated the most revenue were Lazio's - up to $130m
Manchester City and Lazio tokens had dropped the most in value - down 70% since launch
the tokens of two clubs, Inter Milan and Turkish side Trabzonspor, had increased in value more than Bitcoin over the past year
"Fan tokens are being traded more actively than you'd expect for this type of fan-engagement product," Protos director of news David Canellis said.
"Generally, small crypto-currencies like these fan tokens can be incredibly volatile due to the small amount of people who want to trade them.
"Speculators know this, so I would consider much of the trade in fan-token markets to be powered purely by speculators seeking short-term profits."
Individual buyers
Another criticism of the current system is clubs hold on to the vast majority of tokens, rather than selling them to fans, which distorts the market.
The value of tokens held by the top 13 clubs together exceeds $1.9bn - but individual buyers currently hold only $376m worth.
On average, clubs control 80% of their fan tokens' supply.
"Clubs themselves must balance not selling too much at once" Mr Canellis said.
"If they sell too much at once, they run the risk of crashing the price.
"We can clearly see this happening with the Lazio, FC Porto, Santos FC, and Manchester City sales."
Football supporters
Some fan groups are also worried about exploitation.
"My knowledge of the crypto market is sketchy to say the least - and I'm probably representative of a large number of run-of-the-mill football supporters who are not traders on the crypto market," Sue Watson, who chairs the West Ham United Supporters Trust, told the BBC News's Tech Tent podcast.
"It's not regulated, it's not secure and I question what protection clubs have in place for their supporters."
The Socios app has club-token information organised like other crypto marketplaces, with price fluctuations displayed and prominent buy and sell buttons.
But Max Rabinovitch, from Socios, said the system was designed to reward fans who held on to tokens rather than trading them.
Meaningful perks
Offers such as "permanent 5% discounts in the digital shop, possibilities to win tickets" were more important to fans than making "five or ten bucks" selling their tokens, he said.
"There's no value to be had from trading," Mr Rabinovitch said.
"If you want to gather up loyalty points for a team, if you want to vote for the team, you always have that token to use.
"The entire point is buy it and hold it."
But he wants clubs to offer more meaningful perks than votes on minor issues such as "what song should we play in the stadium as the players walk out".
Digital animations
Another way clubs are getting into crypto products is through NFTs or Non-fungible Tokens. Manchester City, Rangers and Juventus have all issued official NFTs, with some spending tens of thousands on these unique digital images and videos with built in code representing ownership. One, millionaire NFT collector Mike Bousis, said he had spent about $40,000 on five of these.
"When I saw the release of Manchester City NFTs, I had to have it," Mr Bousis said.
"I always liked Man City - I like the team, their flash and pizzazz - and I thought it was a cool thing to buy and to have.
"I own it - it's mine, I can display it anywhere I like - and it's a piece of art."
The National Basketball Association (NBA), in the US, was one of the first sports organisations to sell NFTs, offering video clips of basketball games. But Protos research shows the value of these TopShot NFTs has dropped by 90% since early 2021.
These are increasing in popularity, with $142m spent by fans since their launch, in April 2019, Protos data shows.
And sale prices have stayed steady all year.
But football writer Martin Calladine said supporters should be cautious about investing in NFTs - and fan tokens.
'Making money'
"The thing about crypto-currency is it's unregulated," he said. "It's the only product being endorsed in football which has no legal oversight. "Don't buy into it because you sense that it could be a way of making money. "Some people are going to be making a lot of money out of it - but it almost certainly isn't going to be the average football fan on the street."
BBC
