The 74-year-old has been viewed as the world's No.1 football agent based on commissions.
He has brokered deals including ones that took Gareth Bale to Real Madrid and Jack Grealish to Manchester City.
Barnett has often been the one defending his clients in the press, especially Bale against Real Madrid supporters later in his career at the Spanish club.
Speaking in 2022, Barnett said: "Bale deserves a grand farewell from the club. And no, we have not yet decided on the future.
"He speaks Spanish. What happens is that people who do not know him say that he does not. Stupid and ignorant people who have not spoken to him. It's a shame.
"He has always been very happy in Madrid, he arrived as a young man, his children were born in the city, he has donated a lot of money to Spanish hospitals for Covid (a reported 400k) and he loves the city and the club, and I think the parasites are the ones who speak ill of him."