The goalscoring midfielder has already been the subject of a bid from Burnley this summer while Nottingham Forest are thought to be keen suitors as well.
Whittingham's departure from Cardiff could have been made more likely by Tuesday's news that the Bluebirds' financial problems are not yet over.
Manager Dave Jones is again working under a transfer embargo and players have been asked to defer bonus payments and appearance money.
Celtic have emerged as another interested party, with or without a deal that would see Glenn Loovens return to Cardiff, as speculation grows over the future of Aiden McGeady.