Matthew Upson has again signalled his desire to leave Birmingham in January, saying he believes he no longer has a future at the club.
The 27-year-old, who is out of contract in 18 months, has already been linked to Chelsea, Liverpool and Newcastle.
Upson's agent Barry Neville said: "Matt has made it quite clear that he sees his future elsewhere."
Blues boss Steve Bruce is reluctant to release Upson and said it would take "one hell of an offer" to let him go.
"Hopefully we can make Matt an offer he can't refuse"
Birmingham are six points clear at the top of the Championship and Bruce sees Upson as integral to their push for promotion back to the Premiership.
"I have said to Matt if we are promoted, that we would want to build our team around him," said Bruce.
"Hopefully by the summer we can be in the Premier League and can make Matt an offer he can't refuse.
"We've got to be strong enough when the transfer window opens.
"Where can I go and get a left-sided England centre-back who is 27 of his quality? I don't think I can."
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Upson has refused to sign a new five-year deal at Birmingham and has already stated it would be in the club's best interests if they maximised his transfer fee by selling him, "the sooner, the better".
The England international is rated at £7m and was linked with a move to Liverpool before he ruptured an Achilles tendon in April.
He has now returned to fitness after a lengthy lay-off with pelvic and Achilles problems.