But HITC Sport understands that Derby County also want the left back.
However, Fox is yet to agree personal terms with The Owls and Championship rivals Derby County are all set to match their bid.
Fox has come through the ranks at Charlton and has over 100 appearances under his belt. He has been tracked by a host of clubs with both West Ham, Aston Villa and Birmingham all making enquiries over the summer.
Charlton Athletic's Morgan Fox in action with Brentford's Sergi Canos
Now Wednesday have stepped up and agreed terms with Charlton and HITC Sport understands that the player is now due in South Yorkshire to talk terms and undergo a medical.
But Derby have not yet accepted defeat in the chase for Fox and are ready to make a rival bid, in an attempt to snatch him from under the noses of their rivals.
The 23-year-old Welshman is yet to win a full cap for his country, but has been included in a number of squads and is expected to be capped by Chris Coleman this year.
Charlton Athletic's Morgan Fox celebrates scoring their second goal
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