Vitor Pereira said he was taken by "complete surprise and without any warning" after confirming his time as head coach of Nottingham Forest has come to an end effective immediately.
Pereira signed an 18-month contract at the City Ground in February and guided Forest to Premier League survival and the Europa League semi-final.
There was a break clause in his contract for June, and it is understood Forest informed the Portuguese boss they wanted to go in a different direction with the expiry looming.
Reports suggest Oliver Glasner is set to take over following his exit from Crystal Palace at the end of last season.
Vitor Pereira has left Nottingham Forest with immediate effect. Photo by Malcolm Couzens/Getty Images. Pereira said in a statement: "Today marks the end of my journey as head coach of Nottingham Forest.
"I want to say a sincere thank you to everyone connected with this incredible football club.
"Although this decision came as a complete surprise to me and without any warning, I fully respect the club's right to make the decisions it believes are best for its future.
"Naturally, I am disappointed and saddened. I truly believed in what we were building together, and I leave with a sense of pride in everything we achieved over the past months.
"Together, we enjoyed a memorable end to the season. We secured the club's Premier League status, reached the semifinals of the Europa League, and created moments that will stay with me forever.
"Most importantly, I saw a group of players grow in confidence, belief and togetherness.
"Watching them develop, fight for one another and represent this club with passion has been one of the most rewarding experiences of my career.
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"I would like to thank my coaching staff, the players, everyone behind the scenes, and especially the supporters. From my first day, your passion, loyalty and unwavering support made me feel incredibly welcome. You were with us through every challenge and every success, and I will always be grateful for that.
"I leave Nottingham Forest with no bitterness or resentment -- only respect, gratitude and wonderful memories.
"Football is full of unexpected moments, and while this chapter has ended sooner than I expected, I will always look back on my time here with pride and affection.