"To give the club the best possible opportunity of securing the positioning and investment necessary to take it to the next level, at what is an important time in its history, its present ownership has determined that it is in the best interests of Newcastle United and its fans for the club to be put up for sale."
The club said they were willing to listen to offers that would be capable of "delivering sustained investment" towards achieving their ambitions in the top flight.
Newcastle's 52,000 capacity St James' Park is close to full most weeks with the club enjoying a passionate fan base.
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But Ashley, a 53-year-old billionaire who founded British retailing group Sports Direct, has had an uneasy relationship with Newcastle fans during his decade in control of the North East club.
Newcastle have been relegated twice since Ashley took charge in 2007. After winning promotion to the Premier League last season, Rafa Benitez's side are ninth in the standings with 11 points from eight games.
There was tension between Benitez and Ashley during the close-season with the Spanish manager suggesting he was not receiving the backing in the transfer market that he needed.
Ashley's lawyer Andrew Henderson told the Newcastle Evening Chronicle that the goal was to sell the club before the end of the year.
"Our intentions are to see if the club can be bought by new ownership by potentially Christmas.
"That would allow seriously interested parties to put themselves forward. It would allow a diligence period to carry on then hopefully by Christmas a sale could conclude," he said.
"Assuming the right person can be found to take the club forward it’ll be about how payments are structured, how much would be payable on day one and how much would be payable further down the line.
"There is a real genuine desire from our point of view to put the club in the best possible position going forward. If that means additional funds being available in the short-term we would seek to accommodate that," Henderson added.
(Reporting by Hardik Vyas and Simon Evans; editing by Clare Lovell)
