Leeds chairman Andrea Radrizzani has expressed his shock at Garry Monk's sudden departure as head coach.
The Sky Bet Championship club issued a statement on Thursday afternoon announcing Monk's resignation had been "reluctantly accepted" by Radrizzani.
The Italian, who took full control of Leeds on Tuesday, had planned to trigger a one-year option to keep Monk at Elland Road with a view to negotiating a longer contract.
But Monk has told Leeds he will not be staying beyond the end of his contract next month and has left with no compensation due.
"Shocking news from GM. We were keen to do 3 years deal. We never receive any request from him and his agent. No regrets, we did our best #mot," Radrizzani tweeted in response to a local journalist.
If Radrizzani's statement was very matter-of-fact then it is not a reflection of the mood at Elland Road, where staff were surprised by the news.
Monk had just enjoyed a fruitful first season in charge and took Leeds from perennial mid-table fodder to play-off contenders until their charge faded over the final month - something Radrizzani noted himself on Wednesday.
It was a shock when Monk agreed to work for former owner Massimo Cellino a year ago and a greater surprise that he lasted a season without getting the sack. That he leaves of his own accord two days after Cellino sold his shares to Radrizzani is a curiosity impossible to ignore.
Monk was expected to pen a new deal, at least for 12 months, and it is understood he was at Elland Road on Wednesday and gave little clue that a resignation was coming.
''Leeds United can confirm that we have received Garry Monk's resignation from his role as first-team manager," Leeds said in the statement.
''We are shocked and disappointed by Garry's decision but his resignation has been reluctantly accepted by chairman Andrea Radrizzani.
''Andrea made it clear to the media yesterday that his intention was to exercise the club's option to extend the manager's contract for another 12 months and immediately begin negotiations for a longer-term deal.
''Mr Radrizzani has met with Garry twice since taking over at Elland Road earlier this week and during the second meeting yesterday it became clear that Garry was considering life beyond Leeds United, as at no time did Garry wish to discuss terms for a longer contract.
''Following that meeting yesterday Garry's agent requested that the option was not exercised and his resignation was received this morning.
''Whilst we are deeply saddened by Garry's decision, there is no individual bigger than our club and we will now begin a process to identify and appoint a new head coach to take the club to the next level.
''We thank Garry for his contribution during his time at Leeds United. We are disappointed that we could not continue on this journey together.''
At the Riverside Stadium Karanka worked alongside Victor Orta, who is expected to be appointed as Leeds' new technical director after stepping down as Middlesbrough's head of recruitment on Wednesday.
Alan Pardew and Nigel Pearson, who are both out of work, have also been mentioned.