Bradley impressed for Portsmouth last season after joining the club on a free transfer from Hull City.
The commanding central defender made 39 appearances for Portsmouth and his performances have not gone unnoticed among clubs higher up the football pyramid.
The likes of Birmingham City and Sheffield United are thought to be admirers of Bradley having watched him on several occasions last season.
Portsmouth are bracing themselves for offers for Bradley, with the player entering the final year of his contract, and a number of clubs are waiting in the wings hoping to lure him away from Fratton Park.
"We are aware that a number of clubs are interested in Sonny and are keeping tabs on his situation," a spokesman for Bradley's agents New Era Global Sports told Sky Sports.
"I am not surprised clubs are keen on Sonny as he was outstanding for Portsmouth last season and if an opportunity comes up that is right for both Sonny and Portsmouth I am sure we will look at it, but he will remain focused and committed to the club until he is told otherwise."