Nigerian midfielder Mikel has been involved in a transfer dispute between Chelsea, Manchester United and Norwegian club FC Lyn Oslo, where he has been playing.

"Chelsea is aware that at least one copy of a private and confidential letter from FC Lyn Oslo and Manchester United to FIFA dated May 18, 2005, that made unfounded allegations against Chelsea in the case of John Obi Mikel, has been leaked to the media," Chelsea said in a statement on their Web site.

"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."

According to a BBC Radio Five Live reporter who had seen the correspondence, United wanted FIFA to consider imposing a transfer ban on Chelsea, with whom Mikel trained in 2004, as well as barring them from certain tournaments.

Lyn Oslo and United lodged a complaint with world football's governing body last May over Chelsea's pursuit of the 18-year-old Mikel.

Mikel signed for Manchester United in April 2005 but the deal was thrown into doubt when the teenager said he had been put under pressure to sign a deal.

He then travelled to London and said that he really wanted to play for English champions Chelsea. Mikel turned up in Norway in September and resumed playing for Lyn after several months' absence.

"For the record, Chelsea was asked by FIFA for a response after this letter was sent to Chelsea in May 2005," the English champions 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.

"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."