Duff has made more than 200 starts for Cheltenham in all competitions
Club captain Shane Duff looks set to leave Cheltenham after allowing a deadline on a contract offer to pass.
The 28-year-old defender is the last of the Robins' players offered terms by manager Mark Yates to make his mind up.
Yates told BBC Gloucestershire: "As of Friday 5pm Shane had turned down the offer, we'll still talk to him, but we're looking elsewhere.
"He's got something come up that he likes the sound of potentially - it's not in this country."
Duff has been at Cheltenham since joining the club as a trainee 2000, following in the footsteps of his older brother Michael by establishing himself in the Robins defence.
He won a cap for Northern Ireland under-21s in 2003 and the only other club he has played for was Evesham Town in a brief loan spell during the 2001-02 season.
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