Fox, 22, has come up through the ranks at The Valley, and has established himself as a regular first team player in the Addicks' first team under Guy Luzon.
Charlton have produced the likes of Carl Jenkinson, Joe Gomez, Diego Poyet and Jonjo Shelvey in recent years, and after Fox's promising performances for the club, he could end up following those four players to the Premier League.
The Mirror report that Aston Villa, Newcastle United and West Ham United are interested in signing Fox and are keeping tabs on his progress in South London, whilst Manchester United also have the youngster on a shortlist of potential signings after Luke Shaw's potentially season-ending injury.
As a result, Charlton are allegedly hoping to get Fox tied down to a new contract before the January transfer window, before one of those Premier League teams makes a move for the former Welsh Under-21 star.
Aston Villa signed one left back over the summer in Jordan Amavi, but could look to add another as competition, whilst Newcastle desperately need a left back after injuries to Massadio Haidara and Paul Dummett, with Fox having the potential to fill the role for some time.
West Ham may look for a left back if they lose Aaron Cresswell - a player reportedly on Manchester United's shortlist to fill in for Shaw - but Newcastle appears to be the most logical destination for Fox, if he chooses to leave Charlton rather than sign a new contract.