Colchester striker Jamie Cureton has handed in a transfer request.
Cureton was the United's leading scorer last season with 24 goals and won the Championship's golden boot award.
The 31-year-old still has one year to run on his current contract and it is understood the club have been negotiating an extension with him.
Sources close to Cureton have told BBC Essex he is concerned the club's ambitions do not match his and he has handed in a written transfer request.
Cureton only moved to Colchester 12 months ago, although he had spent part of the previous season at Layer Road on loan from Swindon.
In total, the former Norwich, Bristol Rovers, Reading and QPR striker has played 56 games for United, scoring 31 goals in the process.