The president of Sporting Lisbon has launched an outspoken attack on the investor funds which have bought “economic rights” in scores of footballers in Portugal and elsewhere, arguing the funds are a “menace” and a “monster” that undermine clubs’ finances, football’s integrity and pose a risk of increasing match-fixing.

Bruno de Carvalho, speaking at the Soccerex conference in Manchester, called for Fifa and Uefa to regulate more robustly the funds that in recent years have rapidly acquired stakes in players in South America and Europe, particularly Spain and Portugal, where the practice is rife. De Carvalho, 42, a lifelong supporter who was elected Sporting president in March 2013, said that his club are refusing to fulfil the contract the club signed with Doyen Sports, the offshore fund, which claims it is entitled to 75% of the £16m Manchester United paid last month for Marcos Rojo.

Sporting have repaid the €4m which Doyen originally loaned the club for the 75% share in Rojo’s economic rights and 65% of another player, the Moroccan striker Zakaria Labyad, but the club are refusing to pay a further €11m, which would equate to 75% of Rojo’s transfer fee.

Doyen, a fund with undeclared investors and owners based in Malta, which has in recent years bought large stakes in players’ economic rights at major clubs in Portugal and Spain, insists it is entitled to the 75% of the Rojo fee, and De Carvalho said he expected Doyen to sue for the money. He argues, however, that the contracts the club signed under the previous management were in breach of the Fifa regulations, which prohibit third parties from buying “the ability to influence” clubs’ team selections or transfers.

De Carvalho said the contract the club originally made with Doyen was invalid, because it gave the investment fund the right to “manipulate” the club’s decisions. He claimed that Sporting did not want to sell Rojo, who played for Argentina in the World Cup final, but Doyen offered the player for sale over the club’s head, “to Manchester United and all the world”.

“One of the rules is that the funds cannot be engaging with the management and not manipulating the management, and they did it,” De Carvalho said. “That contract means manipulation. It means engagement with the management. It’s not a contract – it’s null.”

Asked if Rojo himself had not wanted to leave, De Carvalho said he had no issue with the player: “He was very happy to be in Sporting but he felt the opportunity to come to Manchester was very good.”

Speaking more generally of Doyen and other funds which buy stakes in players for a share of their future transfer fees, De Carvalho described the funds as “a menace to sports and for football”. He listed the dangers as a lack of transparency because the funds are mostly based in offshore tax havens with undeclared owners, a blight on the finances of clubs because they do not receive all the money for selling players, and argued that they add to the risk of match-fixing because funds could own stakes in players competing for different clubs against each other in the same match.

He said that when he was elected the president at Sporting, who are 51% owned by their supporters, he arrived to find the club’s finances were “a disaster”, they were €500m in debt, and the majority of the players were owned by funds, some having bought 95%, 90% or 80% of the players’ rights.

“I am against funds where we don’t know where the money is coming from, and who try to manipulate football,” De Carvalho said. “Many times there is similar owners from the funds and gambling companies, so match-fixing is the worst fear now for football. Everybody’s seeing the problem.”

Now under his presidency, he said, Sporting will not sell players’ economic rights, and he called for regulation by the governing bodies, arguing that the practice is so widespread that it is impractical for Fifa or Uefa to ban it. “I think now it is a monster,” he said. “A monster that is living in almost all the clubs, so now I cannot see how [it can be banned]; only regulate it is the solution. We need to have a discussion very serious with everybody very quick.”

Uefa has in fact pledged to ban third-party ownership, with the president, Michel Platini, viscerally against the practice of players being “owned” by offshore investment funds. Fifa has a working party examining the issue, which met last week, but Uefa may proceed to ban the practice in Europe if Fifa does not act soon enough.

Doyen Sports responded by arguing that it provides clubs with “much-needed” money, linked to the future value of players, which enables them to hold on to players and compete, citing Atlético Madrid as “a perfect example”, doing well “in part [due] to Doyen’s support”.

Regarding the dispute with Sporting over the 75% stake claimed in Rojo’s transfer, a Doyen spokesman insisted the contract was valid: “We categorically do not manage or influence the player and we ensure that is written into every contract we have with the club. This is the first ever issue we have had and it is with a specific president who now wants to renege on a bona fide contract that his club has signed. We welcome taking the matter to court.”