Chelsea confirmed the player was undergoing treatment in London in a letter to the Nigerian Football Association but failed to release him for independent assessment by Nigeria's doctors.

NFA spokesman Ademola Olajire said on www.bbc.co.uk today: "Mikel's gross act of indiscipline and lack of commitment towards national call-ups is totally unacceptable.

"Since Mikel prefers his club over his country, we can only wish him the best of luck."

Mikel was dropped by coach Berti Vogts from the Nations Cup squad that defeated Niger 3-1 last weekend.

He has also not responded to recent calls to play for the country's U-23 side.

"The fact that both coaches do not need him in their squads means his continuous invitation to the national teams is of no relevance," Olajire added.

"The national coach said before the game against Niger that Mikel did not show any sign of commitment that he wanted to play so he left him out.

"The national U-23 coach Samson Siasia also says he does not need Mikel in his squad since he preferred to play for Chelsea rather than Nigeria.

"Siasia is embarrassed that Mikel, who played under him at the FIFA World Youth Championship two years ago, is being disrespectful and disinterested in playing for his country."