The study is designed to provide an accurate measure of the intrinsic antiviral activity of DTG 50mg twice daily versus placebo both administered with the current failing regimen in a randomised double-blind phase to Day 8. This will be followed by an open label phase with all subjects receiving DTG 50mg twice daily with an optimized background regimen (containing at least one fully active drug) until subjects no longer derive clinical benefit or until DTG is locally available. Thirty subjects will be randomized (1:1) to receive either DTG 50mg BID (Arm A) or Placebo (Arm B) with the current failing regimen for 7 days (RAL or ELV should be discontinued prior to dosing at Day 1). At Day 8, subjects from both arms will enter an open label phase and receive open label DTG 50mg BID with an optimized background regimen containing at least one fully active drug The primary analysis will be conducted after the last subject enrolled has completed the randomised, double-blind phase at Day 8. Additional analyses may be performed prior to study closure when all ongoing subjects transition to locally available commercial DTG.