Aitor Karanka was sacked as Middlesbrough manager on Thursday with the Spaniard's assistant Steve Agnew taking over with immediate effect, the Premier League club said.
"Both parties have agreed that it is in the best interests of the club to make this change," Middlesbrough said in a statement.
The relegation-threatened club, promoted last season after a seven-year absence from the top flight, thanked Karanka for his work over the past three-and-a-half years.
Middlesbrough are 19th in the 20-team Premier League.
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