The invention is directed to a user interface and a method of supplying breathing gas. The interface comprises a pair of non-sealing nasal prongs configured to deliver a flow of breathing gas to a user's nares, a mask arranged about the nasal prongs, a resilient sealing member configured to form a seal with a user's face to allow pressurisation of the user's respiratory system, and a gas outlet in the mask. The nasal prongs have a sufficient bore to supply a gas flow rate in excess of an intended user's peak inspiratory flow rate.
