"There are a lot of clubs who have expressed an interest in John and Blackburn would be one of five in the Premiership we know who would want to sign him," Hartsons agent, Jonathan Barnett, told the Scotland Daily Express.
"Everybody knows John's contract is up next summer but we haven't had any indication from Celtic as to what they want to do.
"John has made it clear that he loves Celtic and living in Scotland but, as yet, we don't know if they are going to offer him a new contract or not.
"Even if Celtic come to us with an offer, I don't expect John to make a decision on his future until the end of the season.
"But, in football, you never know because things change by the minute.'"