Opoku, 26, scored 14 goals in 34 appearances for Carlton this season, and they finished just outside the play-off places in the NPL Midlands Division.
He tweeted: “Sad day but want to say a massive thank you to everyone at Carlton Town.
“Had some great memories over my time at the club and met FFL. Carlton fans and mini millers you hold a special place in my heart, thanks for everything.
“But now I am looking forward to what the future holds.”
Manager Tommy Brookbanks added: "In the three years since me and Mark took over Azza has been superb for us both on and off the pitch.
"On his day absolutely unplayable and a really top guy as well. We will certainly miss him but fully understand his reasoning for leaving and we wish him nothing.
"But success at wherever he ends up and as I said to him yesterday he is always welcome back to Carlton at anytime."
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