A Blues statement said: "Chelsea informed Fifa at the time that these allegations were without foundation.
"The leaking of such fabrications some eight months later does not change Chelsea's position.
"Chelsea Football Club is aware that at least one copy of a private and confidential letter from FC Lyn Oslo and Manchester United to Fifa dated 18 May, 2005, that made unfounded allegations against Chelsea in the case of John Obi Mikel, has been leaked to the media.
"It is outrageous that private and confidential correspondence relating to proceedings before Fifa has been deliberately leaked in this way in an attempt to smear Chelsea's name and, more importantly, to undermine an active Fifa process and pressurise Mikel.
"As far as Chelsea is concerned this whole issue is a matter for Fifa, which is the correct and most competent authority to decide on it.
"Such proceedings involve Mikel, United and Lyn. Chelsea is not a party to those proceedings."