Subjects meeting all inclusion/exclusion criteria and who have successfully completed all protocol procedures at the Screening Visit will enter the two-week run-in period. Following the run-in period, eligible subjects will be randomised (1:1) to one of the following double-blind treatment groups: FF/UMEC/VI 100 mcg/62.5 mcg/25 mcg via the ELLIPTA™ dry powder inhaler (DPI) once daily in the morning and placebo via reservoir inhaler twice daily OR Budesonide/formoterol 400 mcg/12 mcg via reservoir inhaler twice daily and placebo via the ELLIPTA DPI once daily in the morning.