QPR are have agreed a fee with Cardiff for Steven Caulker, who has been linked with Saints.
Former Tottenham centre-back Caulker, rated at some £8million, is a target for the newly-promoted Hoops, but the Cardiff captain, 22, is also attracting interest from the likes of Liverpool, Everton and Saints.
New boss Ronald Koeman could be in the market for a new centre half if wantaway defender DEjan Lovren joins the St Mary's exodus to Anfield.
The only back-up centre halves to last season's regular pairing of Lovren and Jose Fonte are Jos Hooiveld and Maya Yoshida, neither of whom have consistently impressed in the Premier League.
QPR boss Harry Redknapp said of Caulker: We've agreed a fee with Cardiff and it's now up to Steven to decide his next move.
"I know him well from Spurs and he's always been a tremendous prospect. Hopefully that's another one we can get done."