Middlesbrough are to conduct an inquiry into why a number of their players failed to return for pre-season training.
It is alleged Tuncay Sanli, Mido and Afonso Alves were not among the men who reported back to the club's Rockliffe Park training HQ on Monday.
However, Turkey forward Tuncay has been granted an extended break after his international commitments at the end of the last campaign.
But it is suggested that controversial Egypt international Mido and record signing Alves were absent without permission.
A spokesman said: "The players returned to training on Monday. A number of them were missing, some with permission and others without.
"The matter will be dealt with internally."
Goalkeeper Brad Jones and England Under 21 stars Adam Johnson and Andrew Taylor were also excused by manager Gareth Southgate.