Chelsea have issued a High Court claim for £16m against FC Lyn Oslo and their former chief executive Morgan Andersen over the transfer of Jon Mikel Obi.
The Premier League club want Mikel's transfer fee repaid after Andersen's criminal conviction in Norway.
Chelsea say the deal was based on the fraudulent misrepresentation that Mikel had an employment contract with Lyn.
In June 2006 they paid £4m to Lyn and £12m to Manchester United, who had a deal to sign Mikel in April 2005.
"At the time of the transfer, Chelsea, Lyn and Manchester United agreed that the fees paid would be in final settlement of the transfer, any claims related to it and that no further action or comment regarding the transfer would be made," Chelsea said in statement.
"Chelsea has written to Lyn to make clear that because the transfer was based on a fraudulent misrepresentation, now proven by a court of law, the settlement previously agreed is not binding.
606: DEBATE"Chelsea is in contact with Lyn and looks forward to working positively with them to resolve the claim promptly.
"Chelsea would like to make clear that this legal action is against Lyn and Morgan Andersen."
United had claimed they had signed Mikel from Norwegian club Lyn but they later agreed to terminate their option agreement when the player made it clear he wanted to join Chelsea.
Andersen, who had previous convictions for forging official documents, was later convicted of fraud and making false accusations by an Oslo court.
He was given a one-year suspended jail sentence.
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