Chairman Phil Wallace confirmed the 28-year-old will not be signing a new contract after three years with the Boro.
Wallace told the Comet: "Chris told us last week that it was time to be going elsewhere. He's been a great servant to the club and we wish him well for the future."
He added: "All of the out of contract players that we thought would go have now gone. We factored that in some months ago and spent that time lining up new lads, as we look to strengthen the squad for next season.
"We spent months identifying lads that we want to bring in and that is gradually taking shape behind the scenes."