Roy Keane has resigned as manager of Sunderland after 27 months in charge.
Keane, 37, took over in 2006 but chose to step down with the club lying 18th in the Premier League following five defeats in six matches.
First team coach Ricky Sbragia has been placed in temporary charge, assisted by coaches Neil Bailey and Dwight Yorke.
"Roy's decision sums up his desire to always do what is best for the club, despite the club's efforts to keep him," said chairman Niall Quinn.
"Roy deserves huge respect for his contribution and the manner in which he guided the club from the depths of the Championship back to the Premier League.
Niall Quinn expresses his disappointment at the departure of Roy Keane
"His winning mentality and singled mindedness were just what this club needed. Even in his departure he has been more concerned for the welfare of the players and his staff than himself.
"The board has reluctantly accepted his decision and wish him and his family well for the future."
Quinn, who held talks with Keane over three days in an attempt to persuade him to stay, added: "He lifted this place off its knees, he is a tremendous influence.
"Five weeks ago (after beating Newcastle) people were dancing in the streets of Sunderland. It's a funny world football and I hoped I'd never have to do this day.
Keane took charge of Sunderland in August 2006
"I spoke many times about Roy and I being in a partnership and it feels like the partnership has dissolved now.
"I wish him real well in the future. He'll get a bit of time to himself now to re-charge but I think we all know the Premiership hasn't seen the last of him. He's got great things to come."
Keane, who guided the Black Cats to the Premier League in 2006-07, had said in recent weeks that his future may not be certain, although Sunderland have spent the last few days trying to persuade him to stay on.
The Black Cats' latest defeat came against Bolton on Saturday and Keane said after the 4-1 loss: "I ask myself every day if I'm the right man for Sunderland.
"I asked myself this morning and I said I was. Sunday morning, if the answer's no, we'll have to look at it."
Keane spent nearly £70m on 33 permanent signings following his arrival in August 2006, including big-money deals for striker Kenwyne Jones, midfielder Kieran Richardson, defender Anton Ferdinand and a club record £9m on keeper Craig Gordon.
On their return to the Premier League last season they finished in 15th place, escaping relegation by only three points.
Sunderland held a news conference at 1300 GMT in which they confirmed the departure of Keane, who himself issued a short statement.
He said: "I would like to thank my staff, players, Niall Quinn and, in particular, the fans for their support during my time at Sunderland, and I would like to wish the club every success in the future."
Quinn added: "Roy felt he had completed his journey here, and he just felt he didn't want to get it unstuck any further and find ourselves in deeper, darker territory. That's the measure of the guy.
"In situations like this, I am sure nine times out of 10 the chairman is saying how the manager was trying to keep his job. It was the other way round.
Keane's last post-match interview, after Saturday's home defeat by Bolton
"Once he made his decision, I said to him, 'I know you too well, Roy, to try to overturn it now'."
The former Republic of Ireland international is regarded as a legend at Old Trafford where he won seven Premier League titles, four FA Cups as well as the Champions League in 1999.
During his 12-year career at United he earned a fearsome reputation as a no-nonsense tough-tackling midfielder.
Keane also had spells at Nottingham Forest and Celtic, winning 65 caps for his country.
During his first season as manager at Sunderland he led the club to the Championship title.
Keane's contract was set to expire at the end of the season, with talks over a new deal reportedly ongoing.
He is the fifth managerial departure in the Premier League this season. Juande Ramos was sacked as Tottenham manager - Harry Redknapp switching from Portsmouth to replace him - and Alan Curbishley and Kevin Keegan left posts at West Ham and Newcastle respectively.
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