21-year-old Gremio striker Arthur is seen by Chelsea as a potential long-term successor to Cesc Fabregas.
Barcelona who are known to wanting his services may be in trouble as Gremio became angry after their prize asset was seen wearing a Barcelona shirt together with the club representative Andre Cury last Friday.
Brazilians Gremio have hit out at the pictures, saying Barca’s conduct is “unprecedented” and insisted that they will not negotiate with them.
Because of the incident, Barca will have to meet Arthur’s release clause in full if they really want him.
Chelsea are hunting for reinforcements and are keen to lower the age average of his squad.