كتب : Omar AbdelAziz
"This has been the most difficult decision I've had to make in all the time I've been in football," chairman Steve Gibson.
"Gareth has given Middlesbrough Football Club magnificent service as a skipper and, in very difficult circumstances, as manager."
Boro, where plays Egypt midfielder Mohamed Shawky, are currently fourth in the English Championship, only one point adrift of leaders West Brom.
The Yorkshire club also have Mido in their ranks as the Egypt striker is on-loan at Zamalek.