The south coast club are understood to have a £6.8million agreement in place to sign the 26-year-old as part of his year-long loan spell last season and they are confident he will join them this summer.
But Sky Sports reported last week that Tottenham were set for talks with Atletico in a bid to sign Alderweireld themselves, after making their interest known to the centre-back’s representatives.
It is believed Atletico need to buy Alderweireld out of their agreement with Saints for a fee of £1.5m in order to be able to sell him to another club – something they have not yet done within the time-frame set out in the agreement.
And Sky sources understand, therefore, that Southampton will consider taking legal action against Atletico if the Spanish club go ahead and sell Alderweireld to Spurs or anyone else, without settling their agreement first.
Sky sources understand that, at this stage, Belgium international Alderweireld would be happy to sign for either club, as he is keen on a permanent move to the Premier League. He enjoyed his spell on the south coast last term but is also interested in the idea of playing alongside Belgium team-mates Jan Vertonghen, Mousa Dembele and Nacer Chadli at White Hart Lane.
Tottenham are interested in bringing in Alderweireld as they look to sure up their leaky defence. Alderweireld played 26 times as Southampton conceded 20 fewer goals in the Premier League than Spurs last season.
Alderweireld moved to Atletico from Ajax, where he played with Vertonghen and Tottenham attacker Christian Eriksen, but only started 10 times as Atletico won La Liga in 2013-14.
Born in Antwerp, Alderweireld was an Ajax academy graduate who played over 100 games for the club and won the Eredivisie three times between 2011 to 2013. He has 47 caps for Belgium.