Charlton's legal department have been issued with a notice to order owner Thomas Sandgaard to "leave the club".
In the latest twist in a complicated saga that has been ongoing for almost six months, Lex Dominus, the company owned by Manchester-based businessman Paul Elliott, has been granted ownership of East Street Investments, which had been the holding company for Charlton.
There has been confusion surrounding the sale of ESI to Elliott on 25 May, which has been the subject of a number of court cases.
Former ESI majority shareholder Tahnoon Nimer sold Charlton direct to Sandgaard on 25 September after Elliott had failed the EFL's Owners and Directors' Test.
However, Elliott has claimed this contravened a court injunction placed on Nimer that prohibited ESI's sale until his ownership status had been resolved. As Charlton was ESI's only asset at the time, Elliott believes Charlton was subject to the same order.
A court case was due to be held later this month to settle the matter. However, Nimer failed to respond to an 'unless order' issued by the High Court last month which requested documentation surrounding the case and the court has now settled the matter in Elliott's favour.
It is understood Elliott was responsible for payments of around £700,000 being made to maintain the running of the club.
"The former directors of East Street Investments, had no authority to sell anything to anyone, not least agree the sale of Charlton Athletic Football Club to Mr Thomas Sandgaard," said Elliott. "Put simply, it was not their asset to sell.
"We have written to the club lawyers asking them to inform Mr Sandgaard immediately, and the English Football League.
"We never wanted to get to this position, and we certainly do not wish to cause further distress or disruption to Charlton Athletic or its supporters.
"We have attempted to get all parties around the table to discuss a way forward, but, despite our best efforts and Mr Sandgaard's statements to the media about 'doing the right thing' our efforts have been ignored.
"We always argued that we had a legal agreement to purchase the club and that has now been justified by the making of the court order. Mr Sandgaard has been given notice to leave the club immediately.
"We, as always, remain willing to talk, but we also reserve the right to assert our legal ownership of the club."
BBC Sport has contacted Sandgaard, who said he is aware of the letter.