The president of Sporting Lisbon, Bruno de Carvalho, said the club was put under so much pressure to sell Marcos Rojo that the player’s third-party owners turned up to meetings and the directors assumed they were representatives of Manchester United.

The Argentina World Cup defender subsequently joined United for £16m, much to De Carvalho’s distress. “We did not want Rojo to leave. He was an important player for us,” he told the BBC World Football Show.

“The pressure was so big, they started to speak to the clubs and come here to the meetings,” he said of the third-party owners.

“The directors thought they were people from the clubs because they were speaking in English although they were Portuguese. They believed it was a person from a club but it was a person from the funds.”

United said that it was an issue for Sporting’s president and the third-party group and that the Premier League club had no comment.

Similar claims had been made in 2013, when United first attempted to buy the midfielder Ander Herrera from Athletic Bilbao and imposters apparently posing as United representatives attempted to broker the deal.

Fifa last week promised to ban third-party ownership within “three or four years”, in the wake of pressure from Uefa and players’ unions to outlaw the practice. Uefa, frustrated with Fifa’s inaction, had vowed to introduce its own ban on third-party ownership if the world governing body refused to act.

The move came after a Guardian exposé showed that Jorge Mendes, the Portuguese super agent, who brokered the year’s biggest deals, including Ángel di María’s £59.7m move to Manchester United and Diego Costa’s £32m purchase by Chelsea, was serially involved in the third‑party ownership of players.

De Carvalho, who took over at Sporting 18 months ago, has been a vocal opponent of third-party-ownership.

“We don’t know where the money comes from. We don’t know who are the people,” he said.

“The problem is we created a monster, a monster who started to come to football. Without regulations, they don’t help the club. They only give them one, two, three years of surviving but after that they are dead.

“Fifa understand people all over the world don’t want that menace in football. They are almost in every club right now. They are breeding all over the clubs.

“People are very happy to sell a player for £50m, but for the club it is £1m or £2m. And they paid more than that in the salary of one year. Almost all the time you lose money. This is mathematics.”