Aitor Karanka has left Middlesbrough FC the club has announced.
A statement on the club's website said: "Middlesbrough Football Club can today confirm the departure of Head Coach Aitor Karanka. Following discussions, both parties have agreed that it is in the best interests of the club to make this change.
"The club would like to thank Aitor for all his hard work over the past three-and-a-half years and for the success we've had during that time. We wish him all the best for the future."
In a statement Karanka said: "I'd like to thank Middlesbrough for a wonderful opportunity and the players, staff and all the people at the club who I have worked with. I'd also like to thank the fans for their support. This club will always hold a special place for me and I wish everyone connected with Middlesbrough Football Club the very best for the future."
Steve Agnew will step up from his role as assistant Head Coach to take control of first team affairs with immediate effect.
Karanka's last game in charge was the 2-0 FA Cup quarter-final defeat to Manchester City at the weekend.
They have not won a match in the Premier League this year and remain second bottom.