Charlton owner Roland Duchatelet has urged the English Football League to "complete the decision-making procedure" on a proposed takeover.
The Belgian businessman agreed to sell the Championship club to East Street Investments at the end of November.
Duchatelet has been trying to offload the Addicks since the end of 2017.
"Despite several requests from the club, the EFL have not provided a timeline for their decision," the 73-year-old said in a statement.
The EFL replied that it is "impossible to determine a definitive timeline" for the deal.
A spokesperson for East Street Investments (ESI) confirmed to BBC Radio London that the three people hoping to join the club's board have all been interviewed by the EFL.
Tahnoon Nimer, ESI's majority shareholder, the Abu Dhabi Business Development chairman and chief executive Jonathan Heller and the former agent Matt Southall - who is set to become Charlton's new chairman if the takeover goes through - were all spoken to last week.
Duchatelet, who bought Charlton in January 2014, is keen for the deal to go through and says this "period of uncertainty" is "impacting the future of the club".
"We appreciate that the EFL need to do a thorough investigation into any new owners but it has now been a month since the initial conversations and 23 days since the current ownership began submitting documents," he added.
"The process is taking longer than we expected and we would like to urge the EFL to complete the decision-making procedure."
BBC Radio London reports there are no guarantees the deal will be completed before Christmas.
Takeover deal 'unique in nature'
The EFL responded to Duchatelet's statement, saying it hoped to conclude the matter "within an appropriate and reasonable timeframe".
"The EFL has worked with the club, the current ownership and prospective new owners to ensure any change of control is achieved as quickly as is practically possible," a statement added.
"These situations never follow a set format, are unique in nature and the EFL is reliant on the quality of information supplied as part of any submissions.
"The League is committed to applying its regulations and ensuring the appropriate levels of due diligence are undertaken on any individuals wishing to become involved at the club.
"Discussions remain ongoing."
Charlton are 17th in the Championship table, without a win in 10 games, and manager Lee Bowyer is optimistic ESI's takeover will go through.
"It [takeover speculation] has been going on for as long as I have been in charge," the 42-year-old said.
"It is a step closer this time. It is all ifs and buts.
"We have met with the potential new owners and there have been some positive things that have been said.
"Hopefully everyone gets what they want and we move on."