Benitez has publicly complained about Newcastle's spending on several occasions since leading them to promotion from the Championship last season, with Ashley responding that he doesn't have the cash to compete with superpowers like Manchester City, Chelsea and Manchester United.
Now Ashley, who paid £134 million ($178 million, 150 million euros) for Newcastle, reportedly hopes to find a new owner by the end of the year following the formal announcement of his sale plan.
"As one of the Premier League's oldest and best supported football clubs - and for the benefit of its many fans and supporters in the UK and across the world - Newcastle United requires a clear direction and a path to a bright and successful future," a Newcastle statement read.
"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."
Speculation over Newcastle's future was ramped up after the appearance of financier Amanda Staveley at their match against Liverpool earlier this month.
After initially trying to win over Newcastle fans by sitting with them at away matches - the Londoner was often pictured in a Newcastle shirt drinking beer with supporters - Ashley spent far less time at the northeast club once the atmosphere soured in recent years.
He unsuccessfully cycled through a host of managers in a bid to transform Newcastle's fortunes, with Kevin Keegan, Joe Kinnear, Alan Pardew and Steve McClaren among his hires.
Newcastle are currently ninth in the Premier League after Sunday's 2-2 draw at Southampton, but Ashley looks to have realised more investment is needed and he is willing to let a new buyer defer part of the payments to allow funds to be used on improving Benitez's squad.
"To give an incoming owner the maximum possible flexibility to make meaningful investment in the club, including in its playing squad, the sale process will give interested parties the opportunity of deferring substantial payments," the statement added.
"A process will now commence of identifying new ownership for the club that will be capable of delivering the sustained investment in and dedication to the club that is necessary for it to achieve its ambitions." Ashley's impending exit was greeted with glee by two former Newcastle stars Alan Shearer and Rob Lee, who both tweeted celebratory messages.
Shearer's post, showing the cast of American sitcom Seinfeld dancing, was accompanied by the phrase "Just heard.....", while Lee's was captioned "at last".
