Former captain Jonny Howson has left Middlesbrough after eight seasons with the Championship club.
The 37-year-old was out of contract this summer and has not been handed a fresh deal by new manager Rob Edwards.
The former Leeds and Norwich midfielder joined Boro for a reported fee of £6m in 2017 and played in a number of positions including a stint at right-back.
He scored 10 goals in 341 Boro appearances in all competitions.
"His versatility and composure were a joy to behold and as club captain he led by example," a Boro statement said.