Wolves insist record signing Steven Fletcher will not be allowed to leave unless their valuation of the player is met after the striker handed in a written transfer request.
A statement from the club reads: "Wolves can confirm that Steven Fletcher has handed in an official transfer request. The player's frustration stems from the fact that several bids have been rejected for his services, none of which meets the club's valuation."
It went on: "As previously stated, no player will leave Wolves unless the club's valuation is met. Wolves will be making no further comment at this time."
Press Association Sport understands Sunderland have made an improved £12million bid for 25-year-old Fletcher who scored 12 Barclays Premier League goals last season. Fletcher, signed from Burnley for £7million two years ago, presented his written request to Wolves chief executive Jez Moxey and manager Stale Solbakken on Wednesday.
An initial £10million bid from the Black Cats was rejected with Wolves mindful Burnley are entitled to a slice of any future transfer fee.
Fletcher signed a four-year contract with an option for a fifth year when he made the move from Turf Moor.
He netted 10 league goals in his first season at Molineux and a run of late goals, when replacing the injured Kevin Doyle, helped to keep Wolves in the top flight.
But his efforts last season, when again finishing as the club's top scorer, proved in vain as Wolves' three-year stay in the Premier League came to an end.
Fletcher has not played in any of Wolves' pre-season games for the past two weeks because of a minor ankle injury.