Harry Redknapp’s side have already recruited Rio Ferdinand on a one-year deal following the expiry of the centre-back’s contract with Manchester United and are hoping to pair him up with Caulker at the back.
The R's will depart on a pre-season tour to Germany on Monday but they remain keen to close a deal for the 22-year-old Londoner in time for him to join them later on the tour.
Caulker’s contract with Cardiff contains a relegation release clause, meaning he is free to leave the club for a fee thought to be in the region of £8million.QPR have met that price along with Southampton, whose Croatia defender Dejan Lovren is still wanted by Liverpool.
Earlier this month, Lovren admitted his “head was already at Liverpool” after Saints turned down a bid for his services in the region of £20m from the Reds.
New St Mary's manager Ronald Koeman has publicly stated he does not want to lose Lovren, but concedes "a top price" might mean the move goes ahead.
And Koeman has also admitted an interest in Aston Villa’s Dutch centre-back Ron Vlaar, a stand out player at this summer’s World Cup for the third-placed Netherlands.