The Saints had a buy-out agreement in the loan deal which saw the Belgian defender spend last season at the St Mary's Stadium which so far has not been recognised by Atletico Madrid.
A fee of £6.8million would have allowed Southampton to purchase Alderweireld permanently had Atletico not paid them a cancellation fee of £1.5m.
With the deadline for Atletico's payment now passed, Southampton feel they should be able to purchase the 26-year old on a permanent basis but he remains a strong target for Tottenham who have held talks with the player.
Sky Sports reports that the Saints will ponder taking legal action if Atletico do not sell to them with suggestions that Spurs have agreed an £11.5m fee with the Madrid club.
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