English FA Premiership: Chelsea have reacted furiously to a leaked letter in which Manchester United accuse them of acting outside the laws of the game in their attempts to sign Nigerian teenager John Obi Mikel.

In a nine-page letter to FIFA, seen by BBC Radio Five Live, United claim they had a legitimate deal with the 18-year-old and that evidence suggests agents working for Chelsea caused Mikel to disappear days after signing a contract.

Within days, Mikel's agent claimed the teenager had signed under duress and the contract should be ruled null and void.

FIFA have yet to make any decision but Chelsea have said should that case be confirmed, they would have an interest in signing Mikel.

In the letter to FIFA, United and Mikel's Norwegian club Lyn Oslo called for sanctions to be imposed on Chelsea, including a transfer ban and exclusion from certain competitions.

Chelsea are angry that the letter has made it into the public domain, given that the case is ongoing.

A statement released by the Premiership champions, who are 14 points clear of United in this year's title race, read: "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 May 18th, 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."

BBC Radio Five Live reported last night that David Gill confronted his Chelsea counterpart Peter Kenyon and chairman Bruce Buck over the matter in the Old Trafford boardroom following Chelsea's 3-1 win over United in May last year.

Gill is reported to have handed a letter to the pair outlining his grievances with regard to the Mikel case. Kenyon and Buck read the letter in Gill's presence but refused to comment on the allegations.

A week later, Five Live reported, Chelsea faxed United to say they had acted appropriately in the matter and in accordance with all the appropriate laws.

Mikel is currently with Nigeria playing in the African Nations Cup, and he told Five Live he wants to go to Chelsea and likes Jose Mourinho.