Tottenham were also believed to be keen on the 26-year-old defender, who spent last season on loan at the Saints, and have made that interest known to the player’s representatives.
The south coast club are determined to sign Alderweireld permanently and have been in regular dialogue with Atletico.
It is understood that a clause to make the move permanent was written into the loan agreement that saw the Belgium international move to St Mary’s last summer.
Sky Sources are reporting that if Atletico now do a deal with Tottenham or anyone else, then Southampton will consider legal action against the Spanish club.
At the end of June, Southampton head coach Ronald Koeman admitted he was unsure over Alderweireld’s future but expressed his desire to keep hold of him.
Koeman said: “The player likes to stay, we like to keep the player but it’s a little difficulty at the moment with Atletico Madrid, I have a good hope he will stay for Southampton.”
Alderweireld began his career at Ajax, where he won three league titles. In 2013 he moved to Madrid, where he won La Liga and reached the Champions League final in his first season.
He won his first international cap in 2009, and has earned over 40 caps for his country. He made 25 appearances for Southampton last season helping them to secure a Europa League slot with a seventh-placed finish.