Jack Ross has been sacked by Sunderland after less than 18 months in charge of the League One club.
Ross joined Sunderland in 2018 as permanent successor to Chris Coleman but was unable to guide to club to immediate promotion back to the Championship, losing to Charlton in the League One play-off final.
The Black Cats have struggled for consistency at the start of the new season and a 2-0 defeat to Lincoln City has left the club sixth in League One, eight points behind leaders Ipswich Town.
Sunderland chairman Stewart Donald said: "This is a decision that has been made with a heavy heart.
"When we arrived at the club 18 months ago, we appointed Jack because we felt that he was the right man to take Sunderland forward over a number of years.
"Jack has worked extremely hard, and has helped us achieve stability at the club, and I sincerely thank him for his efforts.
"I hope and believe that he will go on to have a successful career in management."
Sunderland have confirmed assistant manager James Fowler will take charge of the team on a temporary basis while the club conducts its search for a new manager.
The Black Cats face Grimsby in the Leasing.com Trophy on Tuesday night before they return to League One action against Wycombe Wanderers on October 19.
'Board felt they needed to make a change'
Sky Sports News' north-east reporter Keith Downie on why Sunderland decided to sack Jack Ross
"It has come as a surprise today but then if you look the bigger picture of the results, certainly since the start of this season and toward the end of last season, maybe it is not too much of a surprise.
"However, both Jack Ross and owner Stuart Donald do have a very good relationship at Sunderland. Ross was Donald's No 1 choice when he put him in charge when he took over the club 18 months ago.
"It is a decision Stewart Donald said it has made with a very heavy heart so it shows he didn't really want to make it but he obviously felt something needed to change somewhere.
"I think Jack Ross would admit this himself - looking at results over the 18 months, while there have not been too many defeats, I think the one thing that has cost Sunderland last season a place in the Championship and sees them in sixth position this season is the number of draws they have had.
"They drew 19 matches last season out of 46 league games and that eventually cost them. They missed out on a place in the Championship when they lost in the play-off final and if they had just turned one or two of those draws into wins they would be back up to the Championship.
"This is where Stewart Donald and the hierarchy at the club feel they should be right now. I think they were willing to allow last season to go if Sunderland had got off to a flying start this time around.
"Results have been mixed this season and, while they have won a couple of games very well, there have been too many draws again and then it has culminated with the defeat to Lincoln at the weekend.
"The board have obviously felt enough is enough and they need to make a change.
"James Fowler, who was assistant manager to Jack Ross, will be handed the reins on a temporary basis but I don't think it will be him who takes the club full-time moving forward."